I own a grip and love it. I was loooking at the simian (tried one on and its a very nice bag), and the prodiscus also but my local shop had grips in stock and I had cash in hand. Haven't regretted it once. If I ever need more storage the prodiscus would be my next upgrade probably. I've seen a Mahal in person and they aren't as huge as the pics make them look and looks like an awesome bag, just don't ever see myself needing that much storage.
The one upside if you don't like the bag you pick is all the ones mentioned so far should be easy to flip in the MP and recoup your investment to try try something else with.