The ms261 is a pro saw vs the ms271 is a farm and ranch mid version. Both are the same 50cc but the pro saw makes .5 more hp is 1.5 pounds lighter. Just because the engine size is the same doesn’t mean they’re identical or will make the same power. Compression, timing/tuning all come into play.
I’m trying to get a buddy of mine to swap his 271 for a 261. He’s 56 and I know he would appreciate the weight difference with cutting all day and it’s probably a little faster. The price is what keeps the average person away but to someone that does 5 plus cords a year, The upgrade is worth it IMO.