You really need to be asking what is best for your needs and driving style.
I run a T34/T3 with a few mods, I have full boost 2 bar by 2300rpm, you are looking nearer 4500rpm for a T4, which will produce more torque higher up, but a hell of a lot less lower down.
If you want to make it really drive able get the inlet ports done around the valve area, man it brings it alive, oh and mine does not even think about getting breathless at the top end due to this, pulling all the way to the rev limit so fast and smoothly I now need to watch the rev counter, the torque difference is amazing.
Torque was one of the things a turbo lump was good at, but spoilt to a degree with T4, as really too big for the power most run, better to sacrifice 10bhp top end to gain 50+ lb ft lower down I say.
A halfway house between turbo sizes would be a cut back T4.
A more modern ecu will give more torque if it has AL properties a lot have like emerald, this is NOT the popping banging sort but in gear AL.
I went for a turbo that will supply enough puff but tried to keep as small as possible, I love it pulls down to 850rpm top gear and pulls with no snatch, in town driving you don't need to go over 1500 rpm, just trundle about all day.
As said a diesel will slaughter the petrol for torque, but lose top end, so it's your choice, steer clear of cheapo turbos made of chocolate though!!
tabetha