I've driven the first gen Mazda3 quite a bit, but not the MS3. For the purposes of your commute, you'll be thrilled with the VW by comparison, unless you relish stop-light drag races along the way. While I've always loved the first gen 3, it's decidedly noisier on the highway, and has a stiff suspension set up. I'd imagine the MS3, with it's even stiffer suspension and larger tires is even more so. The VW, by comparison, has fabulous seats and a great interior, is poised and quiet on the highway, and has a near perfect ride/handling compromise. Of coures, it'll also get way, way better mileage.
So the cars are pretty different, but whether the VW is right for you depends, in part, on how you exploit the "fun" potential of your MS3. The Mazda has sharper handling, defeatable stability control (if it has it at all, I think it was an option), better grip, and way, way more power and torque. If flinging the car at the limit through twisty roads and charging hard down straightaways between corners is an important part of the driving experience, neither the JSW nor the Golf may be for you. And that's not because either car can't keep up with an enthusiastic driver. I find that my Golf is a superb performer that rotates into corners eagerly, takes a relatively neutral set, and is generally more eager to play than most assume it would be. But once you reach the limit, the non-defeatable stability control is heavy-handed about killing the fun. Most people don't explore those limits, but if you own an MS3, I have to assume that you bought it to drive it hard. The VW lets you play up to a certain point, and then pulls the plug. A shame, really, because the chassis is willing in the hands of a capable driver.