The only thing that really matter with the 75 or 27 system is the module. If you have a module of each, they'll both work in the same radio. It doesn't matter which one you get for the Spektrum, But I wold get the 75 version so you can have the spare receiver in the higher bands.
The biggest thing about the Spektrum is you can go at anytime. No more waiting for frequency clips to be able to go out and practise. If you get the M11 (I have both the M8 and M11), make sure the Spektrum system you get says M8/M11 on the box. Otherwise you have to drill out the board in them. It's very easy but why do it when the option is there to not have to do it?