I don't know about the Nextel BBs, but Nextel's Motorola handsets have notoriously loud ringers. If ringer volume is at all important to you, the Curve is probably a better choice. At least it was with Verizon, when I switched from my Nextel i580.

I have also seen people who have upgraded their BB OS complaining about significantly reduced volume following the upgrade, but I don't know if that was particular to one model or another, or a general issue.