Congratulations, both are fine companies!
First, quick caveat: I am a former BCG consultant, from Europe but with consultant experience in the US only.
I would rank both as below MBB, but fairly close - probably #4 and #5 in Europe. My semi-educated guess would be to put Roland Berger ahead of Strategy& in Europe, but reverse this ranking in the US.
Which one to choose? Truthfully, the actual rank and reputation doesn't matter since they are very solid consultancy shops and recognized as such in the UK. I will assume both offices are quite large since in London, so both probably have a number of partners specializing in all the big verticals. If you think of moving to the States, small nod to Strategy&. If not, focus on where you felt best, which people you met and liked most...
One thing most people don't realize is that where you go to school doesn't matter much in the grand scheme of things - and which consultancy you join doesn't really matter much either. A few years post consulting, you would be hard pressed to figure out which of Consultant#1 and Consultant#2 went to MBB vs. RB/Strategy&. Just follow your heart, you will do great. Ah, and congrats again :)