ECU tune by far. Turbo upgrade without tune is not worth much. Just the tune and exhaust will make a big difference. Turbo upgrade, tune and full exhaust is even better. Lots of info related to upgrading turbos to 16T, 18T and various ECU tune on here. Try using google search for 16T upgrade...
Think of it this way: you have a scrawny guy on a bike. He produces a certain amount of power when he pedals. You can either put a bigger wheel on the bike, which would let him go a bit faster because the tire has a larger circumference, so it covers more ground with each stroke, or you could put a giant muscular man who can squat 500 lbs on the bike. Of course, he can produce significantly more power - enough to overpower the gears even. Of course the best way to get more power is to replace the guy (tune) AND the wheels (turbo)
I think the main reason everyone wonders if they can just slap on a 16T is because of reviews like mine that rave about the power increase. The difference is that the stock ecu allows 7psi of boost, which the stock 14T can handle just fine up to redline, and if you slap on a 16T without tuning it you will also be running 7psi of boost, which the 16T can handle just slightly better than the 14T. Now if you run a tune it'll up your peak boost to 14-16psi which the 14T struggles to handle at past 5000rpm, so you feel as if your top end power is weak. Now when you install a 16T, which can handle 20psi of boost, and your tune allows 14-16psi you'll find that it can effciently handle the boost load and easily maintain 14-16psi past 6000rpm, giving you a lot more power in the upper revs.
So just slapping on a bigger turbo without tuning it will show you some gains, but not nearly the gains that a 16T with a tune will show.