Well, I basically started with Asterisk, but ran into some problems (looking back on it, probably because I was a noob to it), so then I switched to OpenPBX (now Callweaver) and it solved most of my problems.
This some of the reasons why they decided to fork (and this is from voip-info.org):
Removal of all Zaptel hardware timer dependencies and use of software timers (either kernel based or generic) in their place.
Replacement of Berkeley DB 1.0 with SQLite 3 as engine for the internal key/value database (aka astdb).
Replacement of internal DSP with superior DSP provided by the SpanDSP library.
Support for STUN, T38 fax over IP and a universal jitter buffer.
Much faster and more efficient dialplan execution.
Much faster and more efficient extension matching engine.
Fixing of bugs which Digium did not acknowledge.
I definitely dont have any beef with Asterisk, but Callweaver seems to be a lot more development (and community-based), most bugs are fixed pretty quickly.