I thought the GF rooms were larger than the Poly rooms? UOG says its 440 square feet v. about 400 for Poly.
A couple of small thoughts on this question. What type of room are you looking at and how important is view to you?
If you're thinking about a garden view, I'd go with the GF. There are, pretty much, no bad garden view rooms at the GF. While most of the garden view rooms at the Poly appear to be fine, and some are apparently spectacular, there are a handful of not so great ones. If you're thinking about lagoon, I'd say it's the complete opposite. You can book lagoon view at GF and be a little disappointed -- one entire building's lagoon views face south, direclty away from the MK. At the Poly, by comparison, just about every lagoon view is great. (Please note, of course, that I haven't actually been in all these rooms -- just going by having walked around the two resorts and read on this forum and tikiman's site, etc.)
It should be noted this is the glass-half-empty way to look at it. That is, this is how I would look at it if my goal was avoiding a bad view in each category. If you look at it from the glass-half-full perspective -- that is, what gives me the best chance of getting a spectacular room -- it's a little different. I think some of the lagoon view rooms at the GF are amazing -- most of Boca Chica, and the ends of Sago Key and Conch Key. Similarly, some of the garden rooms at the Poly are amazing too -- better than the GF. Kind of depends on whether you're a gambler. I'm just saying you can pay alot for a garden at the poly or a lagoon at the GF and be a little disappointed.
A couple of other things. If the MK is your primary destination, I think the GF is ideal -- one monorail stop or a very short boat ride. That makes up a little for the lack of proximity to the TTC. Also, I'm pretty sure that the Poly does not have a fitness center. If theming is important to you, I think the Poly wins.