Its my opinion from what i have read that the 16v are a better all around, general purpose motor and the 20v needs to be pushed harder to get the most out of it. They are a little bit sluggish down low compared to 16v.
Ive never compared so you'll probably need to speak to people who have done the conversion for the results.
Although there are a few 20v's up and running and are making plenty of power.
Theres an ae95 that has 85rwkw which is roughly what the 16v's make at the flywheel, so its up to you.