well, the main advantage of the gtt engine, is the relative simplicity of the conversion, (mind the word relative

) since the engine mounts are the same.
out of your choices, i'd go for the 1.4 16V, mainly cause an engine swap for "only" 8 extra valves sounds like a waste of time and money... besides, the 1.4 16V has 97hp? it's not because you have alot of power, you have to drive to the limit all the time

it's nice to have a power reserve sometimes..
then again, i have no idea whatsoever about those engines..
but i haven't heard of any twingo 1.4 16v conversions, so i'm guessing it's either alot of work, or most people don't find it worth it..
i haven't kept informed bout the MOT prescriptions, but i've looked at dropping in a 1.5 DCI about a year ago.. but those engines were rather pricey back then , it was alot of work and it wasn't allowed on the belgian roads.... so that was of no use for me

on the other hand, according to my calculations back then, i'd have it paid back in 2.5 years, thanks to the price and the efficiency of diesel vs gasoline. and it would be around what.. 100hp / 200Nm? in a car that weighs about 800kg, that's a hell of a lot. and that's before any chiptuning would start
but for a real explanation, i'll have to ask you to wait for other people on the forum, who know more about the engines (and engine swaps) then myself...