The VW GTI's exterior is much more understated and less "boy racer" than the Mazda 3. It's a little boring to some, but I consider it classy and timeless. The panels fit much better than the Mazda 3 as well.
The interior of the GTI is in another league. The Mazda 3 looks nice, but in reality it is really cheap, down to the terrible carpet. The GTI feels better. German quality and engineering is evident everywhere you look.
The Mazda 3 rides very rough considering the handling performance it provides. By comparison, the GTI rides smoother and delivers slightly better handling.
You can get A LOT of features in a $25k Mazda 3. A GTI of the same price doesn't have many of those features. Worse still, many of them aren't available in the USA at any cost. However, the features you get are implemented much better, down to the
The Mazda 3 and the VW GTI both have standard 3 year/36,000 warranties. I was able to extend the GTI bumper-to-bumper warranty out to 4 years/60,000 for only $1055. I was offered a VERY limited extended warranty on the Mazda 3 and it was over $1500.