I don't think you will be pushing it to much at all trying to achieve your goals. to me you just have to think about what will be faster/more responsive. With the 1.5ts you'll spool near 2900 and make power from there on up, with the 3.0ts you won't spool until maybe 42-4300rpm and be making the same power, maybe a little more.
say with the 1.5 you made 380 and with the 3.0 you ran low boost and made 400 I guarantee you the 1.5 is gonna be way faster and fun to drive. lag sucks!! I'd only run the 3.0 or the other turbo you were mentioning if you wanted more than 400whp or more later!!
That being said just stick with the 1.5 and if you want more in the future just upgrade to a bigger turbo later your going to have to re-tune anyways. to me a car should be all about responsiveness not really top end power. You know what they say "torque wins races, horse power sells cars".