Since money is an issue, the I'd say it all boils down to whether you want PASM, xenons and 19" wheels.

If you do, then you've already spent half the difference, and you're going to get the rest back when you sell the car. In other words, the 30 hp upgrade is basically free.

If you don't, then go for the 3.6L Carrera. The engine upgrade is nice, but not so important. What's important is you're getting a 911.