I played the demo on both the 360 and PC. There are very few graphics options on the PC version (may be different in the final product) and it did nothing to stress my hardware. As with ME1 and ME2, this game was really designed for the 360, there may be a HD texture pack for the PC version at some point, but probably not from Bioware. That said though there are some minor noticeable differences between the PC and 360 versions, explosions produce more noticeable debris and the textures are improved, but this does not impact the story or the playability. (Edit: The PC version is at this point not that much prettier over the 360.)
I ended up buying the 360 version, but that was mostly because of the achievements, and the fact that I had played through the previous games on that system. I did score a free copy of ME2 on the PC when I bought Dragon Age 2, and I did really enjoy playing through it a couple times on that platform, but then I had edited .ini files to make my Shepard pretty much a god, and all the money and resources that go with that.
In the end I would say stick with the 360 version, you played through ME2 on the 360 since some of the decisions you made in ME2 will carry over, and possibly make for a more fulfilling experience. Unless they release a DLC like "Genesis" where you can make those key choices without playing through again on another platform (or tracking down save files/editing them).
(Edit: The UI is pretty much the same, and in the demo I don't believe it gave me an option to use the 360 controller on the PC. I would really suggest downloading the demo on the 360 and the PC and judging for yourself which fits you better. I use the same 24" 1080p monitor for my PC and 360, may look different on a large TV.)