Look for reliability (on the internet and here), factor in how much you trust the person selling it,and then factor in the price you want pay. Once you've done that, realize that most phone switches can accomplish all of the same things as far as small PBX systems go, so unless there is a special feature you have to have, I wouldn't base a lot on features. Personally if a customer asks me about another proposed system I am bidding against, I refuse to talk bad about it unless I have direct knowledge and experience using it. i.e. I stopped selling that product because of complaints and issues. Even when that is the case, a good company will mostly refrain from trashing the other system, and instead point out the advantages of theirs and they should be able to send you to a few sites and links on the net with comments about the switch, and refer you to a customer that had the switch to speak with them. On a small system, the rep from the company should be able to answer all of your questions, even technical, with the ability to explain. The best way to tell that the salesperson is trying to make a fast buck is when you ask a few simple questions about the actual operation of the system, and the salesperson defers to a technician. On a system that size a salesperson should have a very good working knowledge of the product. Do your homework! Research the systems as much as you can and pose intelligent questions to the salesperson, if he or she stumbles or give a lot of non-answers they have no idea what they are actually selling you and they have no idea if the product is right for you.
