Considering your situation and the fact that you already have a Brompton, I'd be looking at the Silvertip and the BF, and if the BF fell through for whatever reason, I'd pick up the Silvertip in a heartbeat, no regrets.

You already have a bike you can travel with, so unless you'd rather have a bike that was a bit better than the Brompton at point of destination, you just don't need the packability of the BF.

However, you've obviously drunk the KoolAid regarding Brompton (i.e. you bought one...), so you may be better off with the more cultish BF -- seems like Brompton KoolAid drinkers vie in regard to product evangelizing nearly as much as Brompton owners.

And to be absolutely no help regarding this whole N+1 exercise, if I was arguing in my head about BF vs. Silvertip, the Xootr Swift would be in the running as well, not to mention Montague folders, when considering the other 26" folders you mentioned...

Good luck. The chase and dreaming about a new bike, especially daydreaming spec comparisons, is a critical and enjoyable part of the process...