The 16gxt is basically an old 18g but with better aero, it spools faster and offers more power than an old 18g. So, buy that. The 16gxtr will have even better spool and transient response if maybe slightly less top end.
An old school 20g was only 44lb/min (440chp), the 16gxt is 41lbs/min (410chp), so your talking more power under the curve and a loss of 30hp up top. The new technology turbos are drastically better than the older ones provided better power and response everywhere. Now, I would say the 18gxtr and 20gxtr are going to be a little to big for the 2.0. Particularly without a fully built engine.