I have both Saitos 150 and 180 as well as the OS 160Fx that I fly very hard in aerobatic planes. Billy, trust me the OS160 is more economical than EITHER of the big Saitos. Plus, it'll run well on 5 or 10% nitro whereas the Saitos require 15% or more nitro to run/idle well.
In the real world, you need a 24oz tank for the Saito 180, unless, of course, you plan on put-putting around the patch at 1/3 throttle all day. If that's the case, go with a 120 4-stroke and save the money on the engine and fuel.