Trying to decide on this, the 120 weighs considerably more than the 100 (almost 300 grams more). Would this really make a differenct with a .60 size warbird (spitfire) or would I be adding ounce after ounce of lead to the tail? If I were flying at sea level, I would go with the 100, but being over 5,000 feet up... well you know what happens when you get higher in altitude... less power!

Unfortunately the saito 100 is the most power your going to get at that weight class for a 4 stroke.
Next step up is 91 ys or saito 120 ...very close in weight ! The saito 150 is a 120 that has been stroked and bored so It does not really make sense to get the 120 saito!
The saito 180 is an ounce or so heavier than the 150 and almost the exact same size!...it even has the same mounting dimensions! If your plane's air frame can take the additional torque and vibration generated by the 180...this would be much better than the 120 or 150. 91 fx or 108 OS are also possibilities if you want to go with a 2 stroke.

Saito 180 on a .60 size airplane (roughly 8-10 pounds when completed) hmmmm overkill? Then again there's no such thing as too much power!!! But I do see what you're saying about the 180 being only a little bit bigger than the 120, yet it produces considerably more power. I'll keep that in mind. The plane I'm building is a Top-Flite Spitfire. I don't know if the airframe will be able to hold such a beast of an engine, if I reinforce key areas will it be suitible? I can picture it now, that airplane would be rippin' through the sky.