Aye, those 1.8T and VR6's are fast little hand-picked boogers. Working for a Volkswagen dealership, I frequently get the chance to either drive or ride inside of a multitude of different 'dub sports vehicles. Aside from the W8, the most impressive 'dub I've ever driven would have to be the VW Jetta 1.8T
Though the VR6 has more power, as others have pointed out the 1.8T gets its power a lot quicker than the VR6, being an inline engine with a blower. Volkswagen really knows how to manufacture turbochargers: there is very little turbo lag in these vehicles. They have bypass valves with spiraled intake runners that allow air to get to the engine and exhaust gases to recycle through the EGR even as the turbo begins to spool up.
Even the automatics aren't too shabby. The tiptronic's are quite impressive, but some what redundant unless you're not racing from a dead stop (in traffic, perhaps). I would have to say the VR6 pulls a lot more in 2nd and 3rd and have a lot more power in the middle to high range RPM's (especially middle), but if I had my choice between the two I'd rather have the turbo (the extra 2 cylinders and displacement are nice, but at the cost of horsepower you can get a lot more torque from the turbo 1.8 if you do decide to mod the car). The VR6's are also heavier (speaking of mod abilities).
Now a VR6 turbo... no bets, I'll take that

If I'm not mistaken, the new Golf R32's that we're importing to our dealership from Germany once the production line is approved for the US (I don't know if it's going to be a late year 2003 or an early model 2004). I belive those cars are producing in excess of 230, 240 horsepower. Combined with their low weight and 4 motion AWD (an optional package, of course), we're looking at a Mustang GT's arch nemesis (a la Volkswagen). However, from the 1/4 and 0-60 times being reported by Volkswagen (which I can't tell if they are padded or underrated with the lack of dyno information at my disposal), stock from the factory (especially with the 4motion) might require some minor modifications to get that little forced aspirated booger to "wake up."
We'll have to wait and see on that one, but I've got my money saved (and my at invoice discount

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Anyways, I digress.. back to the SVT vs 1.8T
As others have said, a stock automatic will likely lose to you (by a good car length). A 5 speed manual 1.8 T will take you off the line, but that little straw of a turbo can only spit out so much air before that big mouthed SVT 2.5 (or do you have the 3.0 ?) sucks and blows its way up to 2nd gear (you should be able to catch him).
A VR6 automatic.... even with tip, you should still be slightly ahead (on the track it may be a driver's race). A VR6 manual... having never driven that particular combintaion, I really can't say, but I'm definitely assuming it's going to be a close one in the early gears, but it will very likely lose you in the higher.
I've only driven an SVT once, so I wouldn't rely too heavily on my advice. I say go for it. Numbers can be deafly deceiving. Sometimes the power is stacked in your car's favor, but the other guy wins. Sometimes the power is stacked in the other guy's favour, and you win. I would definitely like to hear how the race goes, though, if you're up to it