In reply to Otis:
I'm gonna throw a little spanner in.
I have 2 sets of half ropes. 60m and 70m. I climb trad with them. Lots will scoff, and rightly so if you like to follow the guide book pitches of yester-year hemp ropes and the unknown above, there is no need for longer than 50's in the UK (or 30m's in many venues). However, if you like to get the meters in.... longer is better.
I like to run pitches together, am blessed with stamina and understanding seconds.
Many UK sea cliff and non grit venues have height where you could use more than 50m. Its not always on the climb, but to the belay behind that the extra is useful. I pick my crag for taking the 70m's, but 60m is frequently used.
Grade doesn't matter to rope choice, what you want to acheive does, you will quickly learn good rope work with longer pitches.
I'd go with 60's as a recommendation. But I guess it depends what you want.
My genesis's (60m) have been great, hard wearing and manageable. Does anyone know of a comparable half rope?
Length advice is well commented on, what about make and model?