Manchester, definitely. I find UCL's engineering department to be less organised and less well funded. Manc has a brand new pilot plant and is the only uni apart from Imperial to have one. Engineering is traditionally UCL's weakest department, whilst being Manchester's strongest. Ignore rankings, especially domestic ones. Satisfaction is bull**** for so many reasons, so don't trust them (most of the time). It is worth noting though, that every International ranking i've ever seen (they don't include satisfaction) ranks Manc ahead of UCL in engineering and QS specifically ranked Manc ahead of UCL by quite a few positions for Chemeng. In fact i believe they're 18th in the world (but dont take that to heart).
Also, Manchester is the most targeted uni with the best career's service in the country and one of the best in the world. It was the birthplace of Chemeng. I also found the department to be really friendly and supportfull. I think the course at Manc is less intensive as well.
I think it's worth noting that if you intend to work in the city after you graduate, UCL is probably better. However, something like 40% of Manc graduates dont go into Chemeng, so going to Manc wouldn't be that much of a draw-back in that sense.