Wow...5' on a 75hp tractor? Talk about UNDERkill.....
Pull type mowers allow you to use a HEAVY mower without the issues of weight hanging on the back of the tractor (like a 3-point mower would add) They're alsojust a tick less manueverable in awkward situations. (It's tough to beat a 3-point mower in an "obstical course") The learning curve for using a trailer type mower isn't too involved. A days worth of use and they become second nature.
Out of the choices you mention, I'd opt for the 10' trailer type mower. Flails do a WONDERFUL job, but CAN require more maintenance and upkeep if mowing in hostile conditions. (BTDT, demo-ed an Alamo commercial grade flail mower last year for hwy mowing.)
If the object is to speed up the mowing, you want the biggest mower that is practical. Again, I'd opt for the 10'er. JMHO, and worth every bit of what you paid for it......