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It helps to understand the intent of each software:

• DesignBuilder is built primarily as a Graphical User Interface (GUI) for EnergyPlus, though it does have some scripting capability.
• OpenStudio is built primarily as an Object-Oriented Application Program Interface (API) for EnergyPlus, though it does have a GUI.

If your goal is one-off models of single buildings, then DesignBuilder builder is an appropriate choice. It gives a friendlier feel to EnergyPlus objects which is nice for new modelers. There is a hefty license fee.

If your goal is to do quick analysis on multiple buildings, and are comfortable with scripting, you will appreciate the utility of OpenStudio and the Building Component Library. The recent inclusion of the OpenStudio command-line-interface makes it a good choice for server-based applications. OpenStudio is free and opensource, which is why many 3rd-party developers use it. The Rhino/Grasshopper plugin Ladybug/Honeybee is particularly nice for thermal comfort analysis and recently CFD. There are still EnergyPlus objects, particularly HVAC systems, that are present in EnergyPlus but not yet OpenStudio.

I find most of the value in modeling is early-phase comparisons of massing, shading, envelope, or HVAC system options, which I can do very quickly with Grasshopper/Honeybee and OpenStudio measures.

Both DesignBuilder and OpenStudio are limited by the lack of controls detail in EnergyPlus, OpenStudio particularly so. I may do early modeling up through schematic or design development phases in Grasshopper/Honeybee/OpenStudio, and then import into IES if I know the controls are going to be really complicated (multiple timeclocks/setbacks, staged humidity controls, zonal and partial air-based system).

Per your specific questions:

• You can model DOAS with radiant slabs or central ground source heat pump in either software. Be careful about modeling radiant slabs in EnergyPlus and consult the engineering reference guide. Setting up the controls correctly is tricky. You may need to do manual sizing.
• Time spent on models: geometry is a big time sink. If you are significantly faster building geometry in DesignBuilder or SketchUp/OpenStudio, that may be the best choice for you for speed.
• Once geometry is built, template objects and/or measures make it easy to get a model running in both softwares. I have OpenStudio measures to apply VAV+boiler+chiller and DOAS+radiant systems, which can run in < 5 minutes. If I have different HVAC systems, I have measures to specify which HVAC systems serve which zones, but need to input that as a .csv, so 20 min? The speed really comes in being able to build/tweak your own OpenStudio measures. You should never manually add and specify terminal unit sizing for a 50+ zone model unless you need to for a code/permit model.
• I'm not sure about DesignBuilder, but in OpenStudio a 150,000 sf rectangular building could be pretty quick. A simple 5-floor core and shell model with mostly defaults I could do in 15 min using measures. Needing to build geometry from floor diagrams and modeling shading adds another 1-5 hours depending on complexity. Specifying construction assemblies, space types, outdoor air specs, schedules, and thermostats is 2-5 hrs. HVAC system is <1 hour, or 6+ if it's a unique/complex HVAC system where I need to manually specify terminal component sizing. Start to finish with documentation is anywhere from 4 hrs for a concept model for massing / shading / envelope component analysis to 16-20 hours for a full design development model, to 30+ hours for permit or LEED model.

It helps to understand the intent of each software:

• DesignBuilder is built primarily as a Graphical User Interface (GUI) for EnergyPlus, though it does have some scripting capability.
• OpenStudio is built primarily as an Object-Oriented Application Program Interface (API) for EnergyPlus, though it does have a GUI.

If your goal is one-off models of single buildings, then DesignBuilder builder is an appropriate choice. It gives a friendlier feel to EnergyPlus objects which is nice for new modelers. There is a hefty license fee.

If your goal is to do quick analysis on multiple buildings, and are comfortable with scripting, you will appreciate the utility of OpenStudio and the Building Component Library. The recent inclusion of the OpenStudio command-line-interface makes it a good choice for server-based applications. OpenStudio is free and opensource, which is why many 3rd-party developers use it. The Rhino/Grasshopper plugin Ladybug/Honeybee is particularly nice for thermal comfort analysis and recently CFD. There are still EnergyPlus objects, particularly HVAC systems, that are present in EnergyPlus but not yet OpenStudio.

I find most of the value in modeling is early-phase comparisons of massing, shading, envelope, or HVAC system options, which I can do very quickly with Grasshopper/Honeybee and OpenStudio measures.

Both DesignBuilder and OpenStudio are limited by the lack of controls detail in EnergyPlus, OpenStudio particularly so. I may do early modeling up through schematic or design development phases in Grasshopper/Honeybee/OpenStudio, and then import into IES if I know the controls are going to be really complicated (multiple timeclocks/setbacks, staged humidity controls, zonal and partial air-based system).

Per your specific questions:

• You can model DOAS with radiant slabs or central ground source heat pump in either software. Be careful about modeling radiant slabs in EnergyPlus and consult the engineering reference guide. Setting up the controls correctly is tricky. You may need to do manual sizing.
• Time spent on models: geometry is a big time sink. If you are significantly faster building geometry in DesignBuilder or SketchUp/OpenStudio, that may be the best choice for you for speed.
• Once geometry is built, template objects and/or measures make it easy to get a model running in both softwares. I have OpenStudio measures to apply VAV+boiler+chiller and DOAS+radiant systems, which can run in < 5 minutes. If I have different HVAC systems, I have measures to specify which HVAC systems serve which zones, but need to input that as a .csv, so 20 min? The speed really comes in being able to build/tweak your own OpenStudio measures. You should never manually add and specify terminal unit sizing for a 50+ zone model unless you need to for a code/permit model.
• I'm not sure about DesignBuilder, but in OpenStudio a 150,000 sf rectangular building could be pretty quick. A simple 5-floor core and shell model with mostly defaults I could do in 15 min using measures. Needing to build geometry from floor diagrams and modeling shading adds another 1-5 hours depending on complexity. Specifying construction assemblies, space types, outdoor air specs, schedules, and thermostats is 2-5 hrs. HVAC system is <1 hour, or 6+ if it's a unique/complex HVAC system where I need to manually specify terminal component sizing. Start to finish with documentation is anywhere from 4 hrs for a concept model for massing / shading / envelope component analysis to 16-20 hours for a full design development model, to 30+ hours for permit or LEED model.