I have owned my GTI for almost 5 weeks now. I am a stereo nut. The first thing was have my buddies at music in motion, install dynaudio threes way components a 7" woofer a 3" midrange, and 1.5" tweeter powered by a JL 300/2, with a JL 10 W7 powered by a 500/1, all feed by a JL cleansweep. And the 10 W 7 pounds harder in this car than it did in my last car. If you are looking for SPL (sound pressure level) huge bass.... Then the smaller the car the beeter, less air to move. Also with the hatch the sub is acoutally in the car, where in the jetta its in the trunk and the air has to come through the back deck and rear seat. Hatch backs are generally better suited for steroes if you ask me.
And the GTI has an impressive tourqe curve which give you 200 foot pounds from 1800 rpms on, and trust me when you on it you are never below 1800 rpms. Handling is great, 3rd and 4th gear are an absolute blast on the high way (mines a 6 speed manual). But even the DSG automatic is fun and is also a 6 speed.
From what it sounds like your looking for, the jetta is the looser go for the gti. Its faster, funner, fewer on the road, and your stereo will sound better.
Hope this helped ya!
Jimmy P