K -Jet's offer better throttle response 'out the box', the stock Digi GTi had a few tweaks to it's fuel maps and ignition timing to help improve MPG etc, BUT get your Digi GTI chipped the maps can be replaced with more aggressive ones and the Digi GTI would be just as responsive and produce more BHP.
To be honest a modded K-Jet vs a Chipped Modded Digi GTi would be pretty much the same - the Digi GTI would have the advantage that the fuel maps and timing could be programmed to suit your mods better than a K-Jet as that level of fine tuning just isn't possible on a mechanical system.
On the other hand if your serious about your mods you really should consider twin carbs or throttle bodies - in which case it really doesn't matter too much as your working with the same basic block but replacing the whole fuel / intake system.
So in answer to your question, if you planning to go as far and say DCOE 40/45's then it really doesn't matter too much - if not If say go for a Digi as there's more fine tuning available and more than likely you'll fine one with lower milage as they are newer.
That's only a brief overview, but I hope it helps !
