I just ordered my 8320 yesterday after deliberating between that & the Pearl. I played with both for quite awhile in the stores. I just wasn't picking up the keyboard on the Pearl easily - to me (old guy@40) the Curve was super easy. That coupled with the larger screen for video / browser and the 3.5mm headset jack and it was a no brainer for my usage needs.
I was also told - but you never really know how accurate the sales people are - that the Curve will automatically use WiFi VoIP and not hit your airtime without you're having to do anything, whereas the Pearl had to be told to do it...? I don't know if that's really true - but again significant for me because I have a LAN at home and at work, so that feature is a plus for me.
Regardless - check them out in person first. I had to go to a few stores to find ones that fully worked etc. It was well worth the effort.
Also - I will add that T-Mobile let's you tether to your laptop if need be, whereas some other carriers will charge extra for that. One additional factor that led to my 8320 decision.