BrentR: I've done Skype on both networks and found VF to be a bit better. XT had similar latency but the voice quality wasn't the greatest. Still very usable. I was using E71 on XT and N86 on VF, both were using Nimbuzz as the VOIP app.
To make that a fair test though , ithink you should try the same app on both phones.
I could be wrong, as ive never used a 3rd party SIP client much.