I guess the answer to this would depend on where the OP is based and what academic track they are hoping to follow. UCL is undeniable in reputation, however in the US not as many students/employers may be as familiar with it as in the UK. Some in the States confuse the name for UCLA! In terms of academic/career track, then if in UK and pursuing STEM, I would say choose UCL hands down. If however the OP is based in the US and was pursuing law, them BU would seem a more appropriate bet.
In terms of Artful Loungers comparison, I would actually say BU is a better bet than BC (presuming you mean Boston College, not British Columbia?) as looking at the rankings it does seem to be better:
Boston College QS 631-640 THE 251
Boston University QS 108. THE 78
Unkess, you were perhaps referring more to general 'word of mouth' reputation etc?