There only reason you should chose an Asterisk PBX System From JohnsonCN:

There's no vendor lock in.

The beauty of the Asterisk system is it's multi-vendor openness. 

We install, service, and support the open Asterisk system. Because the Asterisk system doesn't lock you into any one vendor - you can choose the right hardware for the job without spending more than you have to.

Compared to any traditional PBX bid you will generally find our phone systems to cost significantly less and come with greater quality and more features

Instead of nickel-and-diming you to for every feature, our Asterisk system includes just about everything: Voice-mail, voice-mail to email, conferencing, auto-attendants, fax, follow-me forwarding and music on hold - all the features that older style PBX vendors gouge you for.