I've always like Tannoy. I used to have a pair of Mercury F2 speakers, which may have been the successor to the Mercury M line (I'm not sure though). I thought they were fantastic entry level speakers.
Can't say I've got much experience with B&W though. From all I've read, however, I would expect Tannoy to generally offer more balanced sound.
I take it you are buying used...are all of the speakers in about the same condition?
For an old amp, I would tend towards the Tannoy line, as they are easier to drive, as a product line, than most of the Bowers stuff. If ya happen across some Gold 15" FRSD speakers, lemme know...