Hard to say, actually. I like both in certain cases. On some level I'd have to say yandere, but I have the feeling I would most certainly regret saying that later on in this topic.
As it tends to go, it appears that going for the tsundere is usually (understatement?) quite painful at first and even if you do manage to get their attention they're going to be wishy-washy and non-committal about it, like the whole
'Stupid Wiki, i-it's not like I'm editing you because I like you or anything...' thing. Of course, the real joy to that is seeing a typically aggressive girl blushing and trying to act cold and uncaring. There is a certain joy in that. Then again, that only comes from watching - if you were the person on the receiving end of that, how would that feel if that is all you're getting?
But regardless of that, favourite tsundere for me?
^
'Wh..what? I made a few too many, so I thought I'd be nice and give you one! ...the shape doesn't carry any special meaning, got it?'
<s>Wh-why why don't I miss you a lot for-ever~?</s>
As for yandere...
Oh boy, this could be a weird one to try and justify...
While a tsundere would be hard to get at first, I suppose the advantage of the yandere is that you already have her at the start, and that she'd already be into you. In comparison to the tsundere, I suppose it can be said that it's not as much as a perilous journey to win them over. For that, part of me would go yandere.
On the other hand, God help you if something gets in her way. In fact, this is what makes me regret saying I would choose yandere over tsundere. The potential for baggage would be ridiculous. And while the girl in question is all cute and fluffy, what happens when she snaps?
That being said, my favourite yandere?
...yeah, go fuck a needle, Makoto.
So I'm going to take the easy way out - I like both in certain cases. While part of me would lean towards yandere, it's not by that much more.