Don't know much about Mountain Air...but I can tell you one of the tougher requirements to meet in getting hired with FedEx is the 1000 hours PIC time--Especially with the civilian-time-only folks.
The FedEx pilots' website states, in addition to other req's, you must have:
"1500 hours total fixed-wing time as pilot-in command (PIC) or first officer (SIC) including a minimum 1000 hours PIC in fixed-wing jet, fixed wing multi-engine turbo-prop, or multi-engine recip over 20,000 LBGW or combination thereof.
Note: PIC for this purpose is defined as Captain/Aircraft Commander of record, not simply the sole manipulator of the controls."
So I would say go with the outfit that will get you the fastest upgrades and consequently these PIC hours the quickest.
Good Luck!