I left Royal Marine training last year, however I regret not staying in a bit longer, but now I'm on track with joining the Army as a CS engineer in the Royal Signals it doesn't bother me.
The Royal Marines are amazing, and show incredible professionalism at what they do, and if I wanted an infantry career I'd most definitely go with the Royal Marines if you want to be with the best. I like the whole way that training is structured, with the 'commando tests' at the end of it, and other benchmarks to reach during training. (Such as gym pass-out, bottom field pass-out etc) The Royal Marine ethos, spirit and brotherhood that they generate through training makes you feel like a real close-knit family and a part of the elite. CTC Lympstone is an amazing place to train too.
Seriously, you have to be a certain type of character to get through the training though and I found that I didn't quite have it when I joined. I didn't want the green beret enough.
However, promotion can be slow in the RM and there is long waiting lists for certain specializations. And if you decide the Royal Marines isn't for you anymore, you can't transfer over to another regiment like you can with the Army.