I'm building a home in CT, and will be using radiant heat on all floors and a geothermal system.  Would it be better to generate electricity for the heat pump or use a solar thermal system to provide hot water for domestic HW?  I have about 300 SF of roof space to do this?

Solar hot water typically has a "payback" about 2 to 3 times quicker then solar electric in any given area. Some other outside factors can change that a bit, but I have never seen solar electric with a quicker payback then solar hot water.

You can do what I did.

550 sf of unglazed, Swiss stainless collectors.

I have two heat storages. Inside 800 gallons with heat exchangers inside (STSS tank around $3,000 with tanks from Penn.) and 4,000 gallon tank burried, outside.

During the spring, summer, fall, I use the 800 gallon tank.

During the winter, I run the outside, buried tank with low temperatures and use a heat pump to extract heat from it and heat the 800 gallon tank. The outside tank termpatures are low and the panels become efficient at heating it during the winter.

I would increase the size of the outside storage though.

Eric in Seattle, cold, cloudy.

The GSHP can provide some of your hotwater via a desuperheater (DSH). Thus the solar water heating might have a slower payback if you get the GSHP with a DSH.