These are questions with no right answers, so anything anyone says is just an opinion, include myself. Unless jb or one of the T11 guys wants to give some input, it's really just about preferrence.
I wouldn't call either of them better than the other, because they are very similar in my opinion. They both feel as comfortable in different ways. I really love both of the designs, but if I had to choose one I'd go with the Guardians since they were out first and there is more of a biblical/theological look to them which I think is cool. But then again, the Centurions have the swords, giving it a "pirate" feeling.
Hopefully that helped a little though. If you can, get both, I don't know what your budget is like, but they are really both good decks.
I'd go with guardians. They have a dark print design but a very positive theme, angels. Centurions don't really have a theme other than spades but nothing really makes all of the designs fit together. By the way, I own both and I have to say Guardians are awesome and you don't have to wait to break them in to make them handle well.