Start by asking yourself som basic question:
1) what material do you want them made out of? B&B and SVS are made from 304 stainless. This material is brittle which means it will almost assuredly crack or move around quite a bit during its lifetime. And the "stainless" will turn purple from heat about 5 minutes after your start the car and look bad until you take them off and polish them (unless you have them ceramic coated but the coating does not stick particularly well to the 304 stainless). The Belangers are made out of ceramic coated aluminized steel which is much more flexible so you have less concern about cracking and movement. The ceramic coating sticks well to the aluminized and the headers will be silver (not necessarily shiny) for a long time if you do not wrap them. It is claimed that the aluminized runs much cooler under-hood temps than the stainless. I have no idea.
2) do you want a 5 into 1, or a tri-y with step-down collection system? The Belanger has three tubes on each side going to a mid collector with the other two tubes going to a separate mid collector, and then those two mid collectors go to the final collector. Inside the final collector is something called a "pickle" which is a tube in the center of the merge that effectively takes up space. All this supposedly helps keep exhaust gas velocity high and helps bottom end. Here is a picture of the Belangers. You can just see the tip of the pickle:
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http://secureprocessing.net/PartsRack/WebShop/images/ProductImages/Exhaust/Belanger_SRT1.jpg[/image]
Both the B&B and SVS are 5 into 1 designs, which might be better for top end, especially given how short the runners are. Here is a picture I found of the SVS:
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Note how short the runners are (you can get a sense of distance by noting where the turn into the sill is). Because the Belangers have a much smaller final collector they can reach farther past the frame which means the final collector is right up to the turn into the sill. It is kind of hard to see, but here is a picture of the Belangers as they go into the sill:
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3) what kind of construction do you want? You can see from the shot above that the SVS units use slip fits to join the collector to the tubes (I think the B&B slip-fits earlier, but I am not sure, but if not, how in the heck do you get them in). The Belanger is all bolted flanges, I think the B&B uses a flex section after the collector (neither the Belangers nor SVS use a flex section). The SVS and B&B have individual flanges for each exhaust port, whereas the Belangers have a single long flange that is slit into a three and two port division (I'm guessing the stainless B&B and SVS need to keep them separate to allow for installation of the slip fits).
4) what kind of company do you want to buy from? I know nothing about SVS, but even though the Belanger and B&B factories are about a mile apart, they are worlds away. Belangers is a "mom & pop" type small shop with family including the dogs. B&B is in about a 100,000 squre foot building with somewhere around 100 employees. If you call Belangers, someone named Belanger answers the phone. Billy does not answer the phone (although they do have good customer service).
5) are you going to install them yourself? Hopefully someone will chime in about how hard it is to install each system, but I am sure they are very different installs. I would link you to a mini-review claiming the Belangers are an easier install due to the smaller final collector, but it is on the "other" forum so no can do.
Hope that helps. Just stuff I have learned while learning about my car.