Drive the Speed6 first, you may be surprised at how slow they feel stock. They have plenty of potential but with 260hp through the AWD drivetrain they aren't as quick as the MSP, not to mention they are about 35-3600lbs vs 2800 in the MSP. Plus there is the timing chain and VVT actuator which is likely the reason the guy is selling in the first place (the early CX7s and speed 6s had more failures than the later models), and they have rear diff mount issues that are actually easy to fix but most non-enthusiasts don't do them. I like them because they are Mazdaspeeds and almost as rare as the proteges these days, but I have driven plenty of them and unless it is modded over at least ~300whp they are underwhelming cars. They are built in Japan so they are FAR better than the sh*tbox USDM Mazda 6s but they still have some of the common issues the 6s had, mainly with the interior.
Tuned MSP with a forged motor and larger turbo>stock or mildly modded speed6.
Thanks for the reply, I'm glad to get insight from someone who's actually driven an ms6. He doesn't want to sell it, he has to. Hus dad in Korea needs emergency surgery that is going to cost 20k. It comes with an access port v3, AEM cai, turbosmart dual port bov, and a short shifter. Regardless I will be test driving it prior to purchase. He has an offer from someone else $500 higher than mine. He's meeting up with him this weekend. If it doesn't sell then essentially he will accept my offer. I could match the offer I'm just not that desperate or in a hurry to get rid of the msp.