Insulate an attic or a crawl space? Which one would be the most beneficial

I suspect that if it is cold enough to require any heat, even if the furnace only runs a few times a day, that the slab would stay comfortable. If your climate is such that you will go extended times without the furnace, you may be correct that it will feel cool.

Also, if you have south-facing windows, you may be able to get some passive solar gain using the slab. We have an upstairs bathroom with a dark stone floor that gets afternoon sun (from a window that goes down to floor level) in the winter and the floor can be noticeably warm for a while after sunset.

I own a cape with two finished bedrooms upstairs. We have knee-walls like you, as well as a cap space above the rooms (your upper attic).

We had both the crawl spaces (lower attic) and cap insulated. At the time, we were told there are two types of attic insulation: hot roof and cold roof. In a hot roof, the insulation is attached against the roof between the rafters and the air in the attic is considered warm. On a cold roof, the floor of the attic is insulated, making a warm ceiling, and ventilation and circulation occur in the attic itself (air comes in through the soffit and rises to a ridge vent or other ventilation device).

Ventilation is important for your attic and roof, so you'll want to be careful what you block up.

In my house, we used blow in insulation on the floor of the cap, and we used batting and foam board in the crawl space. You'll want to setup a ripping station outside, and you'll cut the foam board into lengths that fir through your crawlspace door. You carry them in in pieces, nail them to the rafters with special nails, and then use insulating/foil tape to seal the seams.