Since we're not too far away, I'd guess our weather is reasonably similar. If you're even considering heating it makes sense to put the 2" foam down under the slab.
A couple of comments have had me scratching my head. For one, there is no reason to run water to the garage for a radiant system in your situation. It will be a closed loop with water/glycol mix, so there's no need to worry about it freezing if the system is off.
When I started my barn/shop build I told the building inspector (who is a plumber by trade) that I was going to use radiant heat for a 24x40' section of my shop. He asked what I planned to use for a furnace, and I told him an electric water heater (no natural gas run to the barn). He said for my purposes (and what sounds like yours) where it doesn't need to be heated to house temps 24/7, a system using an electric holding tank and an on-demand heater (both rated for radiant heat) would be much less expensive. I can't recall the exact dimensions of his outbuilding, but it's something like 20x25 and he says he heats it with that sort of system, and it costs him $30/mo to keep it in the high 50s to low 60s in the coldest weather.
He was back two weeks ago for my insulation inspection (starting to cover the walls, etc) and we talked some more about it. He insulated under the slab with 2" foam board, had spray foam done on the walls (around R22 as I recall since mine will be similar) and then added blown cellulose over the top.
I'm going to use the setup he described, and if it can keep the shop in the 50s when it's -18*F out, I'll be thrilled....that's warm enough for working inside, and having the floor warm will be a bonus.