Bryston 14B SST ot 7B SST

I am trying to decide which one to buy.

There is a small price difference between a 14b SST and a pair of 7B SST.

Which would perform better.

The difference between the two amps is in the power supply.The 7's use smaller capacitors,16 I believe,which provide a faster power supply whereas the 14 uses 4 large capacitors.The 14 also has a lower supply voltage.In terms of sound the differences are subtle but I would opt for the 7's as you have the advantage of running shorter speaker cables.
I opted for the 14bsst. It was a Stereophile class A choice about two years ago. I very much enjoy the sound from this amp. However you certainly can't go wrong with the 7's. Remember though, you will need another power cord and it would be best to run each monoblock into seperate dedicated opposed to powering both off one circuit. IMHO Bryston is a great choice.
Since the 14B is dual mono, I think the short speaker cable (as Mark said) is the tie-breaker. I owned the 7B SST for several years, but I would not run them from separate circuits unless you've verified the ground potential is the same on each. There is something so soothing to the sound when there is so much power available -- effortlessness.