I've driven both. Considered both cars when I bought the MSP.
Folks around here will be biased, but they are VERY, VERY similar in performance when comparing stock to stock. Both have LSD's, they put out almost exactly the same peak power and torque on a dyno. The Spec-V's torque band is a good deal wider, though. That engine is strong on torque. They'll lay down almost identical 0-60 and 1/4 mi times. Definately would be a driver's race in the 1/4.
The Spec V has a few advantages over the MSP- the 6 speed, the torquey motor, more available options and they are available cheaper- the base Spec V omits some stuff like anti-lock brakes and the upgraded stereo, though. The Spec V's motor is NA instead of FI, which should be good for reliability in the long run.
The MSP's rear suspension is (on paper) a better design, although mine was first clunky, now just squeaky with the new bushings, those have been a dissapointment.
The Spec V doesn't respond quite as predictably when pushed- the MSP is an easy car to drive fast, when you exceed the car's limits it behaves in a controllable manner. The Spec V can get a little wild when driven hard- understeer can turn into big time oversteer very quickly and that's hard to control.
The MSP has a better shifter and better steering than the Spec V. It does lack that 6th gear, though, so revs are high on the highway. The Spec V's gear ratios were changed and 1st is a little taller, but its still too short and its hard to not burn up the tires, then ram into the rev limiter in 1st. The shifter isn't very well sorted out, so the gearbox is cruncy and fast shifts are tough without grinding gears.
The interior and exterior of the MSP are nicer, IMHO. In the end it was the looks and interior layout that sold me on the MSP, as well as the better shifter and steering. It was a tough decision, though, and if I were to have the same decision again, it'd be tough again. The MSP's suspension is awesome, but the creaks and clunks and squeaks it puts out all the time have been a let down. Still, I love the MSP- very fun to drive. So is the Spec V if you ask me.
IN the end, drive them both and pick the one you like best. Each has its pros and cons- they're pretty equal.