If we are talking about cosplay wise then it depends. If you want to do an easy cosplay then go for L. If you want to go for a cosplay that is slightly more difficult then go for Light. In my opinion you should cosplay the character you like more.
Character wise I watched half of the anime and both of the live action movies (never got around to watching "L change the world") and from what I saw from those I honestly liked L more. I dont hate Light I actually liked him at first but after awhile L just grew on me more.
Anime Mid Atlantic [Maybe]
Definite 2013 Cosplays
Goddess Madoka - PMMM
Psyche!Izaya Orihara - DRRR!
Meredy - Tales of Eternia
School Uniform Madoka - PMMM
Magical Girl Ranka Lee - Macross Frontier Sayonara no Tsubasa
Fem!Rin Okumura - Ao no Exorcist
Possible 2013 Cosplays
The Pokemon Black and White 2 Champion
Yuzuki Yukari - Vocaloid 3
Serah Farron - FF13