Both cars are guaranteed classics and ultra-desirable cars. In a perfect world, I'd have both; DC5 for wild track events, GTi MK5 for my daily runabout and interstate runner.
GTi (with DSG) is still an efficient manual, but with the smoothness and convenience of an auto. It is also rare; to me the Merc/BMW of the same price is comparatively underpowered and too common (and uncle-ish). Plus they lack the pedigree; 318i/C180K are considered near bottom-rung cars; while the GTi is top of the range (until the R32 is unleashed). No question about the build quality, safety equipment, fittings and durability; it's a car for life.
As for DC5...it's definately the weapon of choice when it comes to mad thrills. Track days, autocross, rallysprints...if you're into those, choosing a DC5 over the GTi is the sane thing to do. Plus it's hard to live without a daily dose of the wild VTEC.
But at this point of time, I'd steer clear of the GTi until the whole AP-WALD issue is resolved.