I used to be a WinMo user and I will NEVER go back, jailbreak or no jailbreak. The iPhone is much more for me than just the two advantages you listed. The entire user experience is about 20 times better.
Yeah, my WinMo phone backgrounded any time I wanted it to, and I could customize it to my heart's delight, but then why did I ultimately strip it back to the bare OS and just a few 3rd party mods?? I'll tell you why. The phone got incredibly laggy and unstable as hell. I couldn't even answer calls half the time.
I think that's the point many people miss when they talk about going to WinMo, Android, or Blackberry so they can have the freedom to mod and background apps. From what I know and have heard, none of those phones do this very well. They get laggy, music gets stuttery, and the OS starts falling apart.
It's rather odd that the one phone "not designed to background apps" does it so much better than the others, once you jailbreak (especially the 3GS).