I guess the best way to describe the difference is to say the the RO Jett is a custom made high end competition type engine where the OS is a mass produced low end generic use engine.  The RO Jett is usually run on tuned pipe but can use a rear exhaust muffler.  It is very rugged and consistent like a clock.  I found it can carry up to 70 ounces but really tops out at 63-64 for best results.  I've run mine long and hard and none of mine show any sign of wear. 
My one and only .46 LA was long to break in but once done started and ran well.  I was never impressed with power output.  It would run out of punch in the wind on a profile in the low 50 ounce range.  Maybe I was just spoiled to more power.  The LA is fine for smaller lighter airplanes and up to higher expert level flying but needs to be left behind for something better for bigger planes and higher level competing.  A great sport flying engine.

Dave

well it sold for a good price. There is a 50 on the 'bay ATM, but I think I'll stick with the OS 46LA-s at $85!