I am going to buy my second guitar and I want an SG or a Strat,
which one should I get?
My budget is 500 to 600.
Depends on what sound you want. If you want good ol' rock and roll that's really deep (AC/DC, Led Zeppelin) anything heavy, go with a SG. If you want that twangy sound that Clapton gives, or the solos of Lynard Skynard, go with a strat. Also, Strats have Whammy bars.

Personally, I prefer the Strat because it has more versitility in sound.
what's your first guitar? and what do you play?
Considering they are completely different guitars, I'd say you have a decision to make son.
Go to a music store. Pick up a strat and play your 3 favorite songs/riffs. Then plug an sg into the same amp, with the same settings, playing the same 3 songs/riffs.

Whichever guitar you think sounds better with the kind of stuff you like to play is the guitar you should buy!
Does being successful truly make you a sell out? Can't you do your own original music and be successful because of it?