I have been y couple of times on both islands, so it really depends on the season. Avoid French crowds in August and Italian crowds in July.
To be honest, I'am half French but I prefere Sardinia.

The island of Corsica:
most of it is forest,
yes, they do have some tiny lousy beaches as well but'll have to take a speed boat to get there... The costal line is just cliffy.

Whereas Sardinia:
-great beaches,
-better hotels, better food
-no nasty French street furniture (you will need a car on both islands)

Meaning this for cosica, they do not have any money to repair their roads, sometimes, it just makes you think of India or Brasil: http://www.team-oldenhorn.net/wp-con...telegraphe.jpg
http://www.google.fr/imgres?q=lacet+...zoom=1&chk=sbg

http://www.google.fr/imgres?q=route+...0nYCUBQ&zoom=1

http://www.google.fr/imgres?q=route+...zoom=1&chk=sbg

They call it "bootlaces" "Lacets", from those pictures, you'll get a first impression... So good luck :-) ^

Anyway, just try both islands, avoid in Corsica just Ajaccio, prefere Saint Florent instead, go to Porto vecchio, Bastia for a walk around between derelicit houses, same goes for Bonifaccio in the south - it has some kind of atmosphere..

Drop over to Sardinia, just go to Costa smeralda.