I wouldn't make a nickel's worth of difference between the two. .063 or .050; if it ain't sharp, it won't cut.
Last fall, I watched a Mexican sawing down Christmas trees take a brand new little Stihl 280 and revving it up, tried to use it like a crosscut saw; just back and forth. This moron had dulled it up by grounding it repeatedly and thought it'uld saw like that....wouldn't take the time (or didn't know how/didn't care) to sharpen it. Now this grower that hires these Mexicans buys 8-10 new saws every year and gives them to his Mexican cutting crew; they last about 1 season after the abuse these crews give them.....I mean you wouldn't believe....scored, burnt-out saws (no premix), worn out bars with big nicks in the rails (rocks, mud, wrong pitch chains, things like that), busted cases and broke handles (comes from literally throwing them into the back of a pickup).....makes me sick to see equipment treated this way. But this grower don't care; got more money than Forrest Gump....doesn't even take them up and repair them at the end of the season...I think the Mexicans carry them off and sell them for what ever they'll bring....