As a GTD owner twice, it's a no brainer to me...... the GTI. Yep if I was buying again it would be the GTI for sure.
Fuel wise for me, my GTD is averaging around 45mpg (mixed commuting and motorway), and from what I've read on here, a GTI should average around 35 (ish) mpg. For an annual mileage of say 10,000 p.a. and with Diesel (round my way) current £1.219 liter and unleaded £1.199 liter, that would approx equate to £330 year more fuel on the GTI (around £27 month).
In my books, £27 a month to drive a petrol 200+ps turbo hot hatch vs a diesel warm hatch is worth the money any day. Also as you say no one knows how much diesel will be penalized over the coming months / years.
Of course what I don't know is the VED bands in Ireland and the car insurance situation which could sway the decision is they are vastly more expensive than the diesel.
Don't get me wrong, I love my GTD, it's a ruthlessly efficient and capable mile muncher, (but dare i say, just a little non involving as a drivers car), if anything, it's almost too refined, but if I had my time again, I would most certainly have been driving a GTI.