I considered buying it on XBLA a few times, but I wanted to support a developer for supporting the Mac, and I didn't want to have to boot my XBox to play it.
The individual levels are generally quite fluid, unless you die an awful lot of times. But the map screens and some of the bosses are slower (ESPECIALLY the one in World 4/Hell). I wonder if it's the extra visual effects (like the fire in Hell).
I wish I had bought it on XBLA. Microsoft's quality control meant it would have been a much better experience. I was actually disappointed with Steam, which is the first time that happened. I guess I assumed they ran strong QC, which they must not.
PS: MacBook Pro, early 2010, 2.53 GHz, 8 GB. Graphics card (integrated vs nVidia) never seemed to make a difference.
However, Binding of Isaac is pretty lagtastic. (But, quite possibly the most fun I've had with a game this year.)