Unless you are going for big power, why would you want to upgrade? The stock one will handle decent tuning power upgrades if wanted. An upgraded turbo won't really get you anything without upgraded fueling and tuning to match, and a clutch to handle the extra power.
Stock BEW injectors flow plenty of fuel, so a VNT17, a 3bar MAP, a clutch and a tune is all you need for 300ftlbs of torque and better mpg. And the BEW bottom end is much stronger than an ALH and won't blink at those power levels. Stock BEW turbos aren't known for their reliability...
I just bought a VNT 17 (Garrett) from TuneMyEuro.com (AARodridguez). I also bought the EGR pipe that is specific to this turbo and the BEW engine (if you are still running the EGR) and a braided steel oil supply line kit that includes the fittings for the turbo and the block where the banjo bolt used to be. Also comes with all the gaskets and new exhaust copper nuts. Cost just over 1000 USD including shipping. Turbo is on sale @ 775.00. Great company to deal with. I phoned before I ordered and H.P. answered all my questions. Ordered on June 20 and had it on June 24 via FedEx.
I will eventually upgrade to a tune for limited EGR function, mild performance boost and bumped up idle. On their website is a comparison of different turbo options that explains the differences. The VNT17 is a more robust design that comes from the PD170 engine in Europe. Can be installed on a stock motor as I will initially do and can support up to Malone Stage 4 with other mods like a clutch, injectors, 3 bar MAP sensor etc. Hope this helps your decision.