According to the test drive of the mazda 6, the msp is quicker than the mazda 6 inline 4 but a tad slower than the mazda 6 v6 model. The v6 model seems very strong. I test drove the performance packaged mazda 6s and they handle very well and have powerful braking. In hard cornering, the msp holds its ground better but the mazda 6s (inline 4 and v6) corner well nontheless. The steering of the mazda 6 is stiffer but the msp's steering is more responsive. In the braking department, you might disagree with me but the mazda 6 seems like it has more powerful braking. The interior of the mazda 6 is very good and the quality is excellent. I can't say it's better or worse than the msp because it's different. The msp's interior feels more sporty and more compact. The shifter on the msp is short and precise and the mazda 6's is a bit longer but feels very good. The shiftronic model of the mazda 6s is better than anything in its class because the feel of the shifting is tight, crisp and accurate. There is little lag time between shifts.
The ride in the msp is stiffer.
Overall, if you are looking for a performance car, go for the msp. If you are more of a family type, the mazda 6 gives you the performance and the interior space.