No differences can be observed with the naked eye when testing the servos with a servo tester, but I’m not sure if they can operate normally when installed on my SR6 receiver.
After checking around, it seems the version without the -180 is actually 120 degrees. Does it say 120 degrees on the side of the servo? If not, you may have gotten a knock-off or just an older version.
120 degree version without the -180 tag in the model number:
This would make sense, since the 180 degree speed is faster as listed on the specs.
They will both work on your SR6, but you probably won’t get full range and speed on one of them. That might cause mechanical issues much sooner considering the differences.