No one has given a definite answer because there isn't one. It's personal preference.
The half diamond can move in and out smoothly without getting caught, making it better for working back and forth over the pins with light tension until the lock opens. This is why it's favoured by a lot of beginners. You might start out differently though, wanting to really feel everything properly as you go along... in which case you're better going with the hook. The diamond is generally used for haphazard picking and is especially useful for quickly scrubbing or rocking the lock into false set and picking out the spools for a quick entry - it doesn't always have enough reach to get the job done but for the most part it's a reliable technique. That was my experience starting out, anyway.
You
can SPP a lock properly with a half diamond, there's nothing wrong with it in terms of feedback and the ability to move smoothly in and out without getting caught is obviously a good thing. On the downside, it doesn't have the same reach as a short hook. You can't angle a diamond the same way as the outer ramp can disturb pins behind the one you're working on. Besides, a hook won't get caught as you work it in towards the back anyway, providing you have good pick control and tension it properly.
So, the short hook is better in my opinion. A diamond is easier for picking dimple locks in a pinch if you don't have a dedicated set available though, which is a big plus. And it can be handy for smaller padlocks when they're being stubborn and not offering much in the way of feedback. The bottom line? Use what you're comfortable with. It really depends on your picking style, but in some situations, one can be better than the other. That's where experience comes in.
