Looking at buying sw20r?

2000cc VS 1600cc (VTEC 16 makes less power)
In NA form, easier to modify the Honda
Turbo route obviously suits the 3S via conversion
 
Handling etc is much different, you'll likely have more fun driving the SW20. If comparing to a DC2R, it is faster all round than a SW20 NA.
 
Take a ride in one & see how you like it I guess. Aesthetically, I think the MR2 looks much better. Horses for courses.

In a straight line there won't be much difference between an NA sw20 and a b16 civic. So no upgrade in that department. If that's what's you're concerned about then stick with the civic. If you want something a bit different then drive an mr2 and see if you like it.

On the track, a stock standard NA sw20 pulls similar lap times to a stock dc2r at Wakefield. Mr2 has less power but can brake harder and has better corner exit speed. Civic would be pretty close as well, bit slower.