Well - let's see..... Spectrum, this was Uncle Clive's third attempt after the ZX80 and the ZX 81 at marketing a computer for the home user and it was third time lucky for him!
For the home user it was state of the art - remember those rubbery keys on the keyboard and eventually the microdrives?
The BBC "B" was I think in a different league because it was sold in the education market to which Acorn clung for many years. The Beeb had a much higher quality keyboard, used a dedicated monitor and could be interfaced to disc drives. I may be wrong, but I think it used two processors: a 6502 as its CPU and another for running screen graphics
IMHO as the machines were really designed for two different markets, comparison is difficult - the Beeb was probably of higher quality, but the Speccie undoubtedly did its job very well. I can't rermember which computer was announced first but Uncle Clive always had a reputation for mass marketing something at a much lower price than anybody else, sometimes by using pioneering engineering shortcuts, sometimes by cutting quality to the bone. He only really came unstuck with the C5!
Thank heavens he was never tempted to design an aeroplane for the masses. Knowing him it would probably have been the first man carrying flying machine to be powered by .... wait for it ..... a rubber band!!
P.P.