Tsunade is old, but while her justu is activated, she has the looks, well not just the looks but the actual body of a lady in her 20's.. so despite the age fact, her body is legitimately younger than Mei's.. its like who has a younger body compared to who is chronologically younger.. also, beauty is in the eye of the beholder and is clearly subjective..
as for jutsu style, Mei's KKG has indeed more versatility in it but Tsunade's healing jutsu and strength are among the cream of the crop.. personally, i rather be a healing tank than a offensive arsenal who can be killed by a slashes..
and above all, i would rather be royalty than a plain commoner.. a stronger commoner, is still a commoner.. while being born to a prestigious clan and having the right last name seals the deal for me..