If you're interested mainly longer bearing life give a full complement bearing some thought. You almost have to make them yourself. It involves grinding access notches in the inner and outer race so you can remove all the balls and the cage and stuff in a "full complement" of balls with no retainer.
The durability can be substantial. Combat buddy Bob Burch did some bearing calculations(he's more of an engineer than I am). The standard bearing is 17mm i.d. by 25 mm o.d. with 16 very small balls, a very small, light bearing. A standard bearing in a similar size engine case usually is about 12-15mm i.d. by 28mm o.d. and probably weighs twice as much. According to the numbers by Bob(side force on the bearing, # of balls, size of the balls, material, rpm, and vibration forces) gave the lightweight bearing an estimated 6 hrs of run time(at around 25,000 rpm). Going to full complement, 22 balls, upped the estimated life to 30 hours.
Per Randy and Lauri's suggestions, if you must replace the bearings get stainless steel bearings with the same class of clearances and a plastic cage. The plastic is light, low friction, doesn't fatigue, and if somehow bits do come off they are very unlikely to damage the engine(unlike a steel cage).