The MS3 looks great on paper but I keep hearing things about torque steer and not being as refined as a Focus ST.
I also hear they respond well to tuning.
The Fiesta ST is very easy to drive fast, it is eager and generally without bad habits. Handling is excellent and the tighter the track, the better it does.
The tuning potential looks bright but the stock turbo is a bottleneck, if you want a giant killer, budget in a turbo upgrade, downpipe, intercooler and pipes.
Handling and braking are impressive straight out of the box. It can be a bit bouncy on rough streets but not too bad and a good track setup isn't that far from stock. If you like long sessions on a road course, a brake upgrade might be in order but there isn't a true track ready kit out there for 15-16" wheels which seem like the way to go for tire selection.
I like mine a lot and with just a few bolt-ons its even more eager with just a hint of torque steer.
It is small but that brings light weight. If you can live with that trade off, you will like it.