I just filled our 2001 Prizm auto(Corolla rebadged) and 2006 Prius.We drive them the same-heavy,pure city driving with the AC blasting all the time(90 here). The Prizm got 23mpg, the Prius got 45.8.We(wife and son) don't drive to get good mpg.
Your 91 sw might be lighter than the current corolla,and it certainly has a smaller,less powerful motor.The current Corolla probably wouldn't get you better mpg than what you get now.
I would bet you would get at least 15 mpg better with the Prius vs a manual trans Corolla. We average 44 mpg with no effort-you would get 50+ easily.Luck,Charlie