Both areas are pretty popular, it depends on what you're looking for in an RP. Casual tends towards shorter posts (usually around a paragraph), but moves much more quickly. Intermediate people tend to post 2-4 paragraphs at a time, but it might be a day to a few days between posts. You're more than welcome to check both out and see what you think.
I'd also add that I've noticed that RPs get more attention if they're from someone who's already involved. People tend to be more willing to join stuff if it's from someone they've seen RPing in other things (and especially people who've helped out by joining their RPs).
Good luck to you with everything, and let me know if there's anything else I can help you with!