I currently use three combos. A 7' MH fast  casting rod, a 7' 6" MH extra fast casting rod, and a 6' 10" extra fast spinning rod. I cant decide between a M moderate or a M fast casting rod. I would appreciate some advice on this.

What do you want to do with it? I'd suggest a med mod for cranks

I think you could do a lot more with the M/Fast.  Including crankbaits.

I use a 7' st croix avid medium fast for cranks small spinner baits and jigs up to 3/8ths.  Its very versatile.  You might also try a mh/mod and have the same type of versatility, 

All depends on what your doing with it.Length will play its part too.

MM a nice addition for smallish cranks and jerkbaits.

I prefer moderate actions for cranks and surface, power chosen to properly load the rod on casting with the weights I'm using.

I used a MF for crankbaits for years before getting a dedicated crankbait rod.