Exactally what it sounds like. They are incompatible, so I must choose.

TropiCraft Pros: It's farther into completion than the Aether II, and it is very complex, adding things I wish were in real minecraft. There are volcanoes, fish, trading villages, etc. Many of the developers are nice and helpful, and I can easily contact them. You don't fall out of the world. No horrible, evil zephyrs. Enormous amazing home trees. I like it more.

TropiCraft Cons: More complex, there are tornados that could break your stuff, mobs that could kill you, less cool ores. Less people know about this mod, so less would play it.

Aether II Pros: Cool, a new twist on an old mod. Plenty of new blocks and multiplayer features. FLYING PIGS AND COWS. Bunnies. Lots of cool ores! Moas!!! Nice plants and trees. More people know about it.

Aether II Cons: Falling out of the world, zephyrs, weirdo cockatrices. Alpha stage so it's very glitchy. I like it less. There is an official server, so people may play there more. I can't talk to developers.