Pretty sure that by now the OP has probably bought the car of his choice, but since I haven't heard back, I'll throw in my $0.02
Coming from a person that has not only driven both, but has OWNED both cars for almost 10K miles, I feel i'm qualified to talk about this.
The Speed6 would be an awesome deal if it had worked as it was designed to. If everything were spec for spec, I think the Speed6 would be a hands down winner. With AWD, sunroof, and GT options that the 2010 Speed3 doesn't offer, it's just got more bang for the buck. Oddly enough, I got almost 28-30MPG in my Speed6 on the hwy, and I've yet to pass 27 in the Speed3. Driving 2 less wheels, this perplexes me.
While the Speed6 could run you mid $30k new, the Speed3 is almost $10K less new. You get what you pay for too...or so would be the case if everything were working the way it were supposed to. If you've researched the Speed6 at all, you should have come across a few VERY detremental issues, one being a clutch that God himself still struggles with, and the infamous studder where the engine feels like it's missing in mid-range RPM's. The studder almost got me killed when passing on a highway, and that event ended my love affair with it. Most (but not all) 2006 Speed6 owners have reported the studder, while every 2006 Speed6 owner that hasn't had (or couldn't get) the clutch TSB done experiences it's joys. I'm not sure if they fixed the clutch for 2007, but I hear that the studder isn't near as common in 2007. All of this is speaking from a stock engine too, without any mods. I don't know if they straightened it out yet, but the Speed6 engine didn't seem too accepting of mods, even such simple mods as a Mazdaspeed CAI.
The 2010 Speed3 does everything it claims to without any BS. Everything works, no studder, much more acceptable clutch. However, do realize there isn't even (currently) a GT option, so no leather/heated seats, no sunroof option (still can't fathom this), no stock HIDs, no AWD (obviously). The navigation is bundled with the "tech package", and the screen can be tough to read because of it's size, especially in "night" mode when the lighting reverses. Even though some have called it tacky, I for one, loved the Speed6's pop-up 7" screen navigation setup. Was it worth the $2000 option brand new? Probably not. But then again, you'd be buying used anyway. One odd mention is that the Speed6 doesn't have a boost gauge while the Speed3 (2010 gen) does. However, the 3 had to sacrifice the temperature gauge to do so, which brings to question if it was an intelligent trade-off or not.
As long as your tires hook, I think the Speed3 feels slightly quicker, although it's not by much. That is, of course, assuming you can hook the tires without spinning and fighting the wheel from TQ steer. It's very acceptable and even predictable once you get used to the car, but it is still there. Coming from the Speed6 to the Speed3, it was VERY noticable at first since I never had to deal with it in the Speed6. It should come to no suprise the the Speed3 is slightly quicker, as it sports the same engine as the Speed6, but weighs almsot 350lbs less (3250lbs vs 3595lbs).
The Speed3 definately has a firmer ride that meant to feel sportier and tighter around the track. In contrast, the Speed6 feels more like a touring car then a racer. I loved the comfortable ride of the Speed6, but the Speed3 is still very acceptable. A big thumbs up goes to the Speed6's seats, as I find the Speed3's seats to be ok for short hauls but will get a lil uncomfortable if you spend hours and hours in the car every day. I'll agree with others that the rear seats in the Speed6 are much more comfortable and spacious for passengers then the Speed3, although the 3's seats still aren't near as bad as almost any 2 door car's rear seats.
Needless to say, the Speed3 is MUCH better for hauling anything besides people. With the rear seats dropped down and a huge hatch to use, you can fit some monstrous things in the back. The Speed6 has reinforced stabilizer bars in the back and the rear seats CAN be dropped but you have to reach around throught the trunk to do it. The bars can be removed and i've heard of a number of people that have did so without any problems. Even if you remove the bars, your space will still be limited in comparison to a hatchback.
Overall, I'm happy with the Speed3. It doesn't have any of the odd hit-or-miss major faults of the Speed6, although some squeeks and creaks are starting to get worse. As I said, if both vehicles did exactly as they are designed without an issue, i'd probably take a Speed6 over a Speed3. However, after reality sets in and so many of the Speed6's unsettling blemishes start to show, the Speed3 just feels like an all-around better car IMO. Whatever you decide, let us know! Good luck with the search.
