Who has had braces and got a regular metal retainer afterwards? Please help!?!

Umm. I don't think there's anything bad with them. I had to get braces twice and both times wore a metal retainer afterwards, and yeah it's annoying and uncomfortable, but really any retainer is like that regardless of the type.
It could just be a preference of the ortho to use a particular type (every dentistry clinic has its own way of doing things after all, I've sat through a lot of debates between my old dentist and current one on the topic of braces and wires and soforth and those debates mainly end up being personal opinion things). Maybe the whole you'll-regret-it part is his way of deterring you from trying it instead? I think you should ask him why he's saying that, he's the only one who can give you a definite answer.