First decide what size engine you want and then look at specs. Does it have a ball bearing supported shaft? To compare an OS blue, sleeve bearing .46 to an OS .46 ball bearing FX is not a supportable comparison. The same is true of any make of engine of equal size. They all have their quirks. For me I go for dependability and size to fit a particular airplane. The availability of replacement parts or a suitable factory service plan is important too. There are engines with chronic carburetor problems as well as engines with a short life that is unrelated to crash damage. Read the magazines. All seem to have engine experts that comment on various types. You can usually write to the authors with questions about any engine you are considering buying.