The first thing you want to do when you want to compare VoIP providers is to find out what that offer services in your area. You may be able to eliminate a couple providers just by looking at their areas of service. When you remove the providers just will not work out, you have to look at features and price.
One of the biggest differences between VoIP providers is the features they offer. Many are very easy to use routers that allow you to use a regular analog signal and digital signal when it is appropriate. Some VoIP providers only allow calls to other VoIP users, and others, allows you to call anyone, anywhere, anytime.
If you make a lot of long distance calls you really want to be aware of the plans and provider you go with. Some VoIP providers do not distinguish between local and long distance calls, so you basically get all your calls in a month for only a service fee. Other VoIP providers do not allow you to make calls abroad, or they charge exorbitant fees for them. So the volume of long distance telephone calls you make, can be the determining factor in your choice for a VoIP provider.
Has a lot of research, just as you want, when you look at any service. If you know who you are dealing with it is much easier to make an informed decision. Once you determine exactly what you need then it's just a matter of matching your needs with what is offered! Good luck, VoIP is definitely the wave of the future and it will only get easier and more advanced!