I'm more excited for Halo 4 because Halo and I go way back. It's been 5 years since we were last able to control the Chief, and this November, he returns. Halo games have always been solid experiences, so there's no doubt this won't be either. But this also isn't like any other Halo game, Halo 4 completely changes the rules and aims to be new while still being worthy of the Halo name. The covenant are no longer the main threat, Master Chief will be fleshed out even more as a character, the gameplay has received plenty of tweaks, and the visuals have never looked this good on the Xbox 360. 343 Industries has big shoes to fill, and by the looks of it, they look like they could use a bigger pair.
Borderlands on the other hand is a much more different experience. It's not as solid as Halo when it comes to the FPS genre, but it's also not supposed to be an FPS, seeing as it also borrows many elements from the RPG genre. The art style has always been amazing, but the new color scheme as seen in Borderlands 2 makes the game look ten times better. Now I won't deny that Borderlands was an amazing experience for me, but it just hasn't earned the status that Halo has.