There are trade-offs on every aspect of a setup like this. Price, for example, is an order of magnitude higher with PRI (cards, setup, monthly), but it generally considered a superior connection. On the other hand, you are limited to 23 inbound + outbound connections at a time. Adding additional capacity comes at a price, as does the ability to call long-distance.
On the other hand, VOIP (in general, and SIP is included in that) is generally cheaper. In many markets, VOIP is delivered on cable TV or on a DS1 (24 channels of data service on, effectively, a PRI). How the service is delivered to your demark will be the determining factor for how much you have to spend. Internet service delivered over a PRI/DS1 line will be the most expensive and be limited to 1.5Mbps. It will also be the most reliable. From there, the delivery system and speed can vary widely, with similarly wide variations in price and reliability.
The other thing to consider is that, if you are using PRI, your phone service provider is the guy that sold you the cable. With VOIP, you can access an entire marketplace of vendors trying to sell you VOIP services over your Internet connection (or connections). The reliability of these guys varies crazily from vendor to vendor. Their ability to provide high quality services is limited by their expenditure on equipment and their access to higher level PSTN vendors.
I just finished up a project for a company here - I told them I’d cut their bill in half and they scoffed. Their $1000+ a month phone bill went down to about $200 a month, and now they can make more than four phone calls at a time. They, like you, were concerned about reliability, so we put in two cable provider’s Internet and set them up with two VOIP providers. If any one of those goes down, everything automatically rolls over to the other. They have a single server that backs to a “warm” backup that they can turn on and replace their live server in a short few minutes.
So, it all depends on what you’re looking for. If you’re a 911 call center, you just have to build in layers of support and choose your technologies carefully. There’s an old saying: “Fast, Cheap, Reliable - pick two.”