Just because a 747 has 4 engines, doesn't mean BA always use all of them...
Sorry, I couldn't resist. Welcome to FT, UKladySE19. Really, either plane will get you there safely. You should pick the one you feel most comfortable with. There is a website out there (if you search here you should find it - or someone might be along soon to post a link) with statistics for airline accidents. If statistics will reassure you, check them out. I suspect you'll find that the 747 has had more accidents because there are more of them out there, and being older they're often flown by less reliable third world airlines.