Title pretty much says it all. I'm looking in to getting one. And only one. Which is your favorite?
I don't know why you would insist on learning only one of the two sleights. They are both very useful moves and they both have their place in any serious card magician's arsenal.
In my case, I find the strike second deal easier than the push off version and I find these deals easier than any version of the bottom deal. However, as others have mentioned, the bottom deal has a lot of utility and is not a move to be ignored.
Why not practice both and see if you can master them both?
What he said.
The second deal has some excellent applications. I use it regularly as a control (thanks Jon Armstrong!), but as a false deal it doesn't have nearly the utility as the bottom deal. By default, when second dealing, you have to know that the top card is one that you like and you can only keep the top card on top. When bottom dealing, you can keep as many cards as you like on the bottom and deal them as necessary.
Personally, I suck at the bottom deal. My mind just hasn't 'absorbed' it yet. So I like the second deal more, strictly because I can actually use the second deal whereas my bottom deal is pathetic and obvious.
So it comes down to .. what do you want to do? Both moves have their applications.
I will probably eventually get both. But right now, I can only get one of them. So I'm asking which I should get first for more immediate use.