As a fresh new look at the franchise, TMNT is an awesome book.
As a continuation of the movie series and the video game, Ghostbusters is an awesome book.
Personally, I pick up both books each month the day they're released, so I'm not sure I could suggest one over the other.
With TMNT, there's a lot more going on. They have the monthly ongoing book, plus the micro-series issues (with the Splinter issue being written by Ghostbusters scribe Erik Burnham), the Infestation 2 books, an annual on the way and a couple of other things.
Ghostbusters just has the ongoing. But even though there's less material available each month, the story pacing is good enough to just have the main book (plus the PCOC files, concept art, and kid/fan art.)
So yeah, buy em both!