Welcome back from your 2 week (forced) vacation, paininthenuts.
I really like the new Mazda6 myself.
In terms of a comparison, there are both pros and cons when comparing the two cars. I've not driven the new 6 so I'll compare them more generally.
Mazda simply has better engine technology than Suzuki. For a small company, they are very impressive with the amount of R&D they do, especially with engines. They alone kept Rotary/Wankel engine going in automotive applications and are even rumored to still be working on a next gen rotary.
Their new SkyActiv engines are nothing short of impressive engineering. They avoided turbocharging or hybrid technology and went with efficient high-compression ratio N/A engines. These new SkyActivs are proving to be very reliable and returning great fuel economy. Mazda also offers a capacitor-based system for energy conservation to boost fuel economy called i-Eloop.
Mazdas also have great chassis and are the most fun to drive in their segments. From compacts to large SUVs, they usually offer the best handling products. Mazdas have been referred to as the "poor man's BMW" by car enthusiasts.
However, Mazda isn't without fault. They are not as durable as their Japanese counterparts. I have no doubt the Kizashi is better built than most Mazdas though I've not been around the new 6 much to say definitively.
Mazdas are notorious for wind noise, road noise, harsh rides, etc. because they don't use too much insulation and prefer a firm suspension set-up over a soft one. This gives them good handling, but a harsher ride. This was exemplified in my former Mazda3 and the noise and harsh ride was the main reason I went for the Kizashi which is much more comfortable and refined.
Mazdas also aren't the safest cars. I don't know about the new 6, but I doubt it's up to the Kizashi's stellar crash test scores.
Basically, I think the Mazda6 has a
much better engine and they offer diesel (which is supposed to be coming to the US soon too). However, they are like Suzuki and do not offer a V6 or turbo anymore in the Mazda6. Right now, there is only one engine for the USDM Mazda6.
Mazda's transmissions are also much better. Their manual is among the best in class and I can tell you from personal experience that Mazda's manual shifter is
a lot better than the Kizashi's. It's precise and notchy in a way the Kizashi's isn't much.
Mazda also avoids CVTs so their autos are more responsive while offering superior fuel efficiency.
That being said, Mazda is like many others and wants you to buy a CUV in order to get AWD. Suzuki offered AWD in their sedans like the Kizashi and SX4. The only recent Mazda sedan to offer AWD in the US was the late Mazdaspeed6. While I personally liked that car, I've heard it's not very durable compared to a Subaru WRX/STI or Mitsubishi Evo.
Given my previous ownership experiences with both Suzuki and Mazda, I believe the Suzuki is better built for durability and will last longer down the line. My old Suzuki Sidekick took a beating offroading and was very reliable and durable. My Mazda3 was treated much better yet was the most problematic car I've owned (though thanks to many Ford-sourced parts in it).
The new Mazdas are Ford-free for the most part and are already showing much better reliability, but their long-term durability still needs to be proven. The other Japanese brands simply have a better track record.
So in the end, both have advantages and weaknesses compared to each other.
However, as you said, Kizashis can be acquired for a good price and they are near unbeatable in value. This is what makes the Kizashi so special. You get so much for the money, especially as a used vehicle. I have a very nicely equipped Kizashi with low miles and I paid damn-near base model Toyota Yaris money for it.
Win.