Every ROM is different in that the developers tweak or add things to the ROM that make it unique to that particular build. Sometimes you'll notice nothing between ROMs, but the code and addons are what set them aside. You'll see clear differences in 2.1 ROMs v AOSP ROMS, particularly the UI included in each ROM and how it performs.
Generally, most ROMs for the Hero are pretty stable, unless otherwise stated by the Developer. CM7 ROMs include nightlies, which are experimental and deemed unstable, but stable enough for daily use in most cases. It's hard to say which ROM is the most stable to run for a particular person because every expects different from their device.
I hope this clears some things up, ask more questions if you need further clarification