Honestly they're both good Russell Group unis, Soton has pretty much been consistently in the top 20 for a long time now (without spending lots of money on publicity/advertising), since well in the early 90s, Birmingham usually does pretty well too, but I think Soton does better overall, especially if you're doing something like engineering.
IMO if you feel more comfortable and at home there then that's great, but was that just on a one day visit or something?
Modules are important-that's what you'll be doing remember.