..... but assuming both will happily give over 200bhp.
The T series turbo was 200hp as standard, and many people have shown that it can happily cope with the strains of a lot more horsepower than that.
The K4 on the other hand, evolved from an engine that was originally designe to be a maximum of 1.4 litres capacity and about 84hp.
Taking it to 1.8 litres and 160hp (either with the VVC or with the turbo) is already stretching it, and whilst some people have fitted a K series 1.8 turbo and tuned them to the kind of power levels your are talking of, it is very likely to prove a step too far to aim for 200hp+.
MG Rover did experiment with a 1.8T in a couple of ZRs, at least one of which was tuned to somewhere around 200bhp. A member of this forum who was a homologation engineer at Longbridge at the time (Rincewind) got to drive and described it as 'a hoot to drive', but told us that MG Rover limited the 1.8T to 160bhp simply for reasons of engine longevity - they considered that any temptation to push it further in production cars would lead to premature engine failures.
Due to the engine being a 1.4 that had been stretched beyond its original design parameters to make the 1.8, meant there were compromises in the components, not least the thin walls of the 1.8 cylinder liners, necessary because there was not the room in the block to take the bigger bore and retain the same cylinder liner thickness as the 1.4. Not only would the liners be vulnerable under strain, but there is also the resilience of the head gasket to consider
Probably going to need to spend big money to safely get the 1.8T to where the T Turbo starts off from.
A 1.8T would probably be more responsive (due to its much lighter weight), but if you want to tune to 200bhp or more, I would think the T series will prove easier to do this with, and more robust.
The biggest drawback to the T series is that it was last fitted to cars built 20 years ago, and people have been relentlessly robbing the engines to put in ZRs in particular for 15 years, so finding a decent condition T series that hasn't already been interfered with (and very likely wrecked) and been driven to the moon and back isn't as easy as finding a reasonably low mileage, good condition 1.8T K series.