I do those races because I'm sponsoring a team of 35+ guys and I like racing with them, I love racing in the city, really like the guys on the team, etc. Also, finding 45+ races is harder. But I'll be going to more of those. There's a Tuesday nighter 45+ here, and one on Sat. AM in Newark. Plus just hanging in the field in the 35+ (which I manage to do in 90% of the races) is an amazing workout for me. Makes me faster. I'll try to mix it up with more 4/5 racing and 45+ where I'm potentially competitive. Me racing with the 35+ is essentially a workout, those guys are over my friggin head for sure.
I would prefer 45+ because those fields are far safer and more experienced.