The HS 55 is definitely a better servo than the s75, I've been using them for years in airplanes and have no complaints other than they are easy to strip (probably not as easy as the s75, though).
Dave/Choppersrule has them in one of his Blades i think, he can tell you better how they handle there, I won't know till next week but I'll bet they will be noticeably better handling, and for sure more reliable.