Much of your decision will be based on price and features. Another consideration is if you have an existing non-VoIP PBX, do you plan to scrap it for the new system, sell it? There are alternatives that allow your existing PBX to access VoIP, like Quintum.
As for pure VoIP, you should look at Avaya, Cisco (really do not like their call manager setup though) and also do a search on SPA9000. This is a low end, low cost, IP PBX, but with all the features of a highend one.