I actually would HIGHLY recommend it. I'm in Year 12 this year and am in the midst of Extension Two mania and I LOVE it! LOL. That is, of course, until something doesn't work and I get bitter and twisted.
The variety of Extension Two is pretty cool. Poetry, movie, speech, critical response.. the list goes on! I'm doing a Critical Response. I guess each 'medium' requires different traits on YOUR behalf. You should go through the outcomes from the syllabus booklet (your teacher will have it) and see what is required. Then, and only then, you can make your decision..
As for 3U - well, again, there is a variety. Compared to the Ext. 2 program, there's A LOT of variety! Haha. I'm doing Revenge Tragedy and am loving it. I'd have never had an appreciation for the RT texts beforehand and, really, as cliched as it sounds, I feel a better person for doing both 3 and 4 unit.
Good luck!