From a programmer and web designer's standpoint forums are easier to code than chatrooms, as Khalo stated.
My preference, I really don't have a major one either, though I normally do form roleplay, mostly at this site, for the simple reason that its easier to find people on a forum than in an IRC or instant messenger that want to roleplay. Also, I find that in the majority of Instant Messenging RPs I've gotten into:
- Involved only two to three people (including myself)
- Had a post order (I post than you post, and so on)
- Were ERPs (Erotic Roleplay)
In an essence my experience in the real-time IRC/IM roleplay is limited to things that wouldn't be allowed here. I would do both if I knew where to look. For now, mainly forum roleplay. forum roleplay is also good if your a busy person: all you need is an hour out of your day to read and write for a forum. IRC/IM requires a good block of time, and the people involved have to be online... its a headache.