Depends on how dirty you are willing to get your hands. 20v (or 4agze) is a pretty easy swap and is not all that involved you can order the adapter harness (not needed for an mr2 4agze) then drop in the 20v then plug it in, even bolts to the old gearbox. The 3sgte is a custom job, you need to make the engine mounts and position the engine precisely for the axles to even. You can order a plug and go harness for it but the work to get to that point is large. I am almost done with my 3sge (no t yet
) celica swap and it was a very involved process and took hours of research before I launched into it. There is a v6 swap as well and that is a very nice swap, good for a show car as it can look very clean and nice with a wire tuck. v6 is also a lot of work.
All perform to different tastes but if you just want outright easy power go 3sgte. The 20v and 4agze perform pretty well fo 1.6L engines and you would probably be happy with the performance bump. If you want a torquey fun to drive car do a v6 swap. Like I said it depends on what you want.