People forget the most important aspect is the OS not the hardware. Also, the only PDA/Phone combo that juggles both acts well is the treo. The PPC side is still more PDA orientated with cellular abilities rather than a good phone and good PDA.
Anyway, the 6600 running 2003 sucks. WM5 is respectable and possibly worth the switch. 2003 is horrid and I really hope you enjoy it but I wouldn't be surprised to hear your complains shortly.
Being limited to only Verizon smartphones is a liability as well. I would say the HTC Universal would be ideal but probably to much. The WM treo is a pretty good device as is down to $300. Oh, and we could have helped you get a good deal on a X51v, you were talking about paying retail, are you crazy? I would say $175 is not a screaming deal, its about what its worth and even to much in my opinion. There are a lot better PDA's out there and I would say that a EVDO bluetooth phone and regular WM would be better suited than the 6600.
OK, sorry, I guess if you have nothing nice to say don't say anything at all. I'll shut up now.