Mazdaspeed MX-5
My MSM would beat my buddies S2K from a stop until I topped out every time. The MSM suffers from bad gearing so the car tops out at about 135mph. Once I replaced the stock restrictive intake, leaky BOV, and rubber throttle body pipe with Flyin' Miata parts, the car was a hell of lot better with a lot less lag.

If she decides on the MSM, then swap in the 5-Speed tranny from the normal Miata.

www.mazda-speed.net <- Good MSM Owners Forum. Could help give her some more info.

S2000
Never owned one, but drove a buddies car regularly. The car is very fun and loves to rev, much like most Honda cars. The suspension is tight from the factory and the car seemed to have less roll than the MSM did stock. While the MSM would take the car off the line and thru most of the gears, the S2K would go faster in the long run so if she's looking for highway speed, the S2K may be her better bet unless she plans on swapping the MSM gear box.