Well I enjoy both, under different circumstances.
First of all, the manga is a lot more intense. It isn't repetitive like the anime (especially in terms of every anime episode having SOME KIND of problem with the Rocket Trio). The storyline is strong, and at some points even very scary/sad. Red actually catches all 150 pokemon. He starts off as a cocky and ignorant little boy who thinks he's the strongest pokemon trainer because he can beat everybody in pallet town... until he meets Oak's grandson. I love the characters and their development.
Let me get this out there, the anime is aimed at a younger audience. It's a lot more childish in terms of a weak plot line (and a repetitive one), in which younger kids won't notice as much.Ash is like the typical young shounen protagonist who faces failure often but never actually gives up. He teaches kids the value of friendship through the amazing relationship he has with his partner, Pikachu. I find the anime is relatively pleasant on a... brainless day. But I've been pretty interested in the new series X/Y out and I've been following along (maybe because I ship Ash and Serena!).
All in all, if you prefer a less childish approach, the manga is for you.
If you don't mind a cute show about never giving up and friendship despite being more childish, then the anime would interest you.
The game series is the best though. Hahahah.