Tests Of Alternate JSF Engine Show Higher Thrust

GE's F-136 has demonstrated a 15% sea level thrust advantage over the Pratt & Whitney F135 at the USAF's Arnold Engineering Development Center.

Additionally, GE is saying that they are doing this at lower turbine inlet temperatures, which would imply lower maintenance costs as well as greater upgrade capability:

The intense battle over powering the F-35 Joint Strike Fighter could be heading to new levels following test results that show the General Electric/Rolls-Royce F136 alternate engine has more than 15% thrust margin against specification, significantly exceeding the power of the baseline Pratt & Whitney F135.

The tests at the U.S. Air Force’s Arnold Engineering Development Center (AEDC) in Tullahoma, Tenn., are the first to officially calibrate the combat-rated thrust of a production-representative F136 at sea level conditions. Although the test program is only a matter of days old, it already appears to be showing greater performance margin in afterburner than expected, says the General Electric Rolls-Royce Fighter Engine Team.

I would note that it is likely that, as with the F100/F110 comparison, that the GE engine is somewhat heavier, which would imply that at higher altitudes the P&W engines would provide better performance.