Well there are bound to be a few lemons in the bunch. That's life. And that's with ANY automaker. I don't care what name you throw out there. Mercedes? Has lemons. BMW? Has lemons. Toyota/Lexus? Has lemons. Honda/Acura? Has lemons. So my apologies to you all that had a lemon out of the bunch, I personally have dealt with the same. But it's no reason to throw an automaker under the bus entirely. I mean when you're making thousands of the same vehicle on end you have to expect there are gonna be some **** ups. I mean it's just like the human race... even god slipped one through here and there that didn't quite make it to every part of the production line... and can we really blame him? I mean come on... he's too busy to worry about a few lemons, let darwinism take care of those few.
Whatever the case back to the topic. I think the MS3 will honestly take the cake compared to a WRX. Now compared to an STi I think is more of a feasible option although the edge goes to the STi; And with good reason.
The MS3 will have amazing fit and finish. Will be more driver oriented and will be easier to live with on a daily basis. I mean think about it. Gas mileage will be better, insurance rates will most likely be better (subjective). The daily driveability will most likely be more livable. And it will be an overall more comfortable car.
The WRX/STi are amazing vehicles don't get me wrong I love them both in their own right. But after owning a WRX it's easy to say the car really shows it's intentions. The car is okay on a daily basis but you really compromise a lot compared to a car like the MS3. The stereo system is so-so. The comfort level is great if you're a purist. And the ergonomics are non-existant.
As for the MS3 it offers a plethora of options that really appeal to anyone. Looking for an out of the box warranty covered track fighter? You got it. Looking for a gas sipping, cargo carrying, people toting wagon with some spunk? It's all there for you. Do you care about comfort? Do you plan to make any long trips in this vehicle? Do you want a decent sound system, great ergonomics, good build quality, upscale options (Navigation, Xenon's, etc.). It's all there in black and blue.
Whatever the case you CAN'T go wrong with either vehicle. I'm sure mazda has a smash hit on their hands with the MS3. But I'm only echoing what everyone else has already said. The aftermarket will most likely jump on the MS3 quick but it still won't catch up to the WRX/STi market. At least not anytime soon.
Personally I would get an MS3. Enjoy it SLIGHTLY stock for a while with the comfort of knowing I have a valid warranty. And then later on after I've really gotten the feel for the car start looking into the aftermarket after giving it some time to develop for this car.
Good luck in whatever you choose.