For the record, I've tested these two engine (OS .50 SX and ST .51) with static tach readings and flying both in identical airframes and I'm sfraid they are not close in power at all. In fact, the ST is only mid field performance-wise amongst the pack of .46s at our field. On an APC 11x6, the ST .51 did 11,800 rpm compared to an OS .50 on the same day which did high 12s on a stock exhaust or early 13s on a $15 Tower muffler that bolts right up. That's a very significant difference you will definitely notice in the air. Unlike the ST, the OS loves to rev and is just as happy on a smaller prop at 14-15k.

The OS is essentially a set and forget engine. Set the high end, set the low end for idle and transition and then fly. The .50 SX, properly set up, has exceptional throttle response. The ST, even with the requisite several hours of spray bar jiggery pokery, will never transition as cleanly or run as smoothly at part throttle as the OS.

If you run them both at full throttle, the ST will probably use less fuel, chiefly because power comes from burning fuel and the OS significantly outpowers the ST. In non full throttle usage, there is no material difference in consumption that I could notice.

In terms of reliablility, at our field, more STs have required re-ringing than have OSs - the main symptom being an inability to start by hand. Bearing reliability is more a function of user care (or lack thereof) and attendant corrosion than brand of engine. Both are pretty dependable engines when used and maintained as specified.

Both the OS and the ST are happy on lower nitro fuel.