I just purchased a Goldberg Ultimate, and I cannot decide what motor I should go with...calls for a 60 to 90 2 stroke or an 80 to 1.20 4 stroke...I can pick up a new OS 91 2 stroke for about $200.00 or in the mid to high $300.00 for a 4 stroke...and yes money does play a little into it..

What would be the major difference between a OS 91 2 stroke and a Saito 100 4 stroke as far as power and performance?

I doubt you would see a huge difference between an OS .91FX and the Saito 100. In fact, the .91 may be a bit stronger than the Saito. I've flown the Goldberg Ultimate with the old OS .91FS, the OS 1.08, and the 1.20 four stroke and all had ample power for my tastes. The .91 may lack a bit in the vertical if you are used to having airplane that hover but it certainly offered better than scale performance. The 1.08 was my favorite, lots of power and lighter than the four strokers. Good luck with yours, they are great flyers. (but lousy gliders, make sure your engine is reliable)