I have been using 7B STs for the last 16 months or so and see no reason to "upgrade", "downgrade" or "sidegrade". From what I read, the new SST series is better. The question is: to what extent? I don't think that the introduction of "new" or "improved" amps by Bryston should be seen as indicative that the previous models are, somehow, flawed.
So if you have the money to buy new, I would see what the best deal is and would go with either the 14B SST or a pair of 7B SSTs. I tend to favour monoblocks for all the obvious reasons: shorter speaker cables, totally isolated channels, probably better heat dissipation from the two separate chassis. So, money being equal or almost, I would go with the monoblocks. If you don't want to spend for new, I would use the same logic. Every new iteration of a product is, somehow, perceived as providing vast improvements in sound. That's fine. But if you go with the older model ask yourself a simple question: how significant are the improvements? Hard to do a side by side comparison, I agree, (and I think that is what you actually want to find out here) I would wager that all other factors being equal, the difference is probably very small. On one side we are told of a revolution a week in power amps, and on the other that they all sound alike given the same power rating and decent noise and distortion figures. The truth is probably that they do sound pretty much alike, with small improvements being made over time. Add to that the fact that improvements can be for reasons other than sound quality (heresy!), such as manufacturing efficiencies, improved reliability and marketing pressures, and running equipment that is not of the absolutely latest generation is not a bad proposition.