Both are the playing the same position. One seems to be under achieving and is paid a lot but is a proven talented, borderline all star while the other has shown he's really good but can't bring it consistently. Both can play very good defense, Gay seems to be better at creating his own offense but Green seems to be more efficient. Both are athletic.
This does not describe either player.
In a league where featured 3s are rare, Rudy Gay is absolutely a borderline all-star, or at least has played so in the past . Questionable effort, poor shot selection hold him back, but keep in mind Paul George wasn't any better offensively last year.
You can count the number of featured SFs in the league on one hand.
Lebron
Melo
Pierce
Durant (not in this order of course)
Iggy
George
(Cue Princess Bride references)
Then who? Deng, Batum, Leonard...they're all at best the third best players on their teams. Rudy Gay and Jeff Green both will likely be the best or second best player on the floor for their teams during large chunks next season. After them it's all specialists or clear second tier players. Gallinari, Parsons, Barnes, Harris..those guys could 'make the jump' next year, but haven't yet and all of them outside of Harris will play a clear third or fourth fiddle coming into the all star weekend.