[Vince]

Hard to say, depends on the situation and how they're implemented. Dialogs are easy to set up and can support a lot of text, in terms of both length and character sets. The big downside is that they are clearly a layer on top of the original game and they miss a lot of customization options. The fact that you're limited to two buttons is also somewhat annoying since some dialogs would benefit from three buttons (yes, no and cancel; previous, next and cancel).

Textdraw provides much more customization and adheres to the original feel of the game but it cannot display a lot of text and it is very hard to do right. Textdraw can look very amateurish in an otherwise good gamemode which kind of sends mixed signals to the players.

So in the end I'd say: if you have lots of text use dialogs, if you want customization use textdraw.