They are completely different. NOC (or pure-forum based in your terms) is all about analyzing posting behavior and relationships between users in these posts. Typical smogon games (OC, or IRC mafia in your terms) allow outside communication, which drastically changes the game. Since players no longer have to share everything they have to say with everyone, it is more about figuring out things via roles and role claims.
It generally comes in a few phases. Early on, one or a few people step up to lead the village, and have to prove their trustworthiness and ability to find mafia members. Once they have sufficiently done this, the other villagers will claim what their role is to the leader (typically you can copy/paste your role pm, so mafia members must create a fake role pm), so that the leader can analyze everything and best utilize the village's roles to find the scum. Typically, at a later point in the game, the village has been whittled down to where they are about on even ground with the mafia(s). At this point, the leaders of each faction try to convince each other to turn on each other so that their faction can stay unharmed and gain the upper hand. Typically in these types of games, there are no vanilla players (players with no special role), so there are many variable actions to use to find scum, kill scum, protect town, etc (and the opposite from the mafias' point of view).
As the two styles are quite different, it can be difficult adjusting from one to the other (particularly in the direction of NOC -> OC). The best way to adjust is to read game and postgame threads to find out what works and what doesn't, and when you feel you are ready, hop in and try it out. Experience is key, a lot of the better players started out making huge mistakes early on.
As for which is better, I much more enjoy OC to NOC. NOC games are typically filled with long posts that you have to keep up with, and it can be hard to keep up with if you're particularly busy and have to read a couple pages at a time. OC is enjoyable because it involves lots of in depth strategy. Additionally, I don't think it is too fast-paced (at least that's my opinion). Generally, being online for a few short periods of time (like being online at 12 PM, 5 PM, and 9 PM, for a half hour each) is enough for your average players (leaders of factions need to be online more often, as they do the most communicating).
I would make an analogy that NOC games are like checkers whereas OC games are like chess. NOC games are much more simpler, and while there is strategy, it is both completely different and much easier to figure out with repetition than that of its OC counterpart. OC is like chess in that there are many different pieces that do different things and you have to formulate a strategy using what you have to beat your opponents.
