Well, to start off with, i would get one of each deck to try them out, because we all have different opinions on handling, design, etc. However, if you were to ask me which i prefer using, i would say that they are pretty much the same. The design is outstanding on both decks, and is far different than any other deck.
On the other hand, you said that you want to try something new. With that in mind, you should look into the Split Spade Lions by David Blaine, as well as the Smoke and Mirrors by Dan and Dave Buck. They are all outstanding and long-lasting cards that will leave you satisfied in the end. I, personally, would prefer Split Spade Lions over the Guardians and Centurions magic-wise, but cardistry wise, it's the other way around.
Bottom line, there is no major difference in quality between Bicycle Guardians and Bicycle Centurions. Its mainly the design that is unique in its own way. Try out all the Theory11 decks and see which ones you like.
-Greg