There are three main routes up Sgurr nan Gillean:
South-East Ridge (Tourist Route)Normally graded at Grade 3, but downgraded to Grade 2 (not sure why!), in most recent SMC Cuillin guide.
West Ridge via
i) Easy Chimney & Tooth Arete (Grade 3/Moderate)
ii) Tooth Chimney (Difficult)
iii) Nicholson's Chimney (Difficult)
Pinnacle Ridge (Grade 3/Difficult)
As to quality of routes
Pinnacle Ridge ****
West Ridge **
South-East Ridge (Tourist Route) **
Pinnacle Ridge is awesome.
Crib Goch is only a Grade 1 scramble, so all three routes up Sgurr nan Gillean are technically harder.
As a newbie, if tackling SnG I would recommend the South-East ridge and/or hiring a guide.