I don't play a lot of free, or well, any at all, so I can't go very in-depth there. As far as casual and advanced go;

Casual's main strengths are having a huge audience, easily excitable players who are ready to jump in. You can also work in a lot of different things and ideas because players can switch and adapt quite easily, provided your ideas are interesting.

Casual's main weaknesses are that because there are so many alternatives players drop more easily and so everyone tends to be less committed. Also don't attempt too complicated engines or plot-lines, because casual players just want to pick up and play. 

Advanced's main strength is that when a game gets off, it offers the best potential for depth and complexity, due to the slower pace and patience of the players. It's also the section where you don't have to worry about skipping a week of posting and can go at it quite relaxed.

Advanced's main weakness is that between the slower post frequency and... Frankly a lot of people writing more words than content, the pacing can be horrible. Some of the hardest to read posts on the forum can be found in advanced, as well as some of the longest intervals between posts. While players won't switch gears as quickly as in casual, this is no less deadly for your game because every so often people will think your game is dead while one post would've saved it. ie, as a GM ALWAYS have IC important elements to reply to for everyone. Also learn to use your OOC far more than your IC.