hi bro. the diff between the SZ 320/ 520/ 720 are merely cosmetics. the flame top of the 520/ 720 are visually appealing but that's about it, its contribution to tone is marginal (what can you expect from a mere laminate top...?). also, the 720 features fancy inlay, if you like them then by all means get the guitar, but remember, this again isn't a tonal feature- purely cosmetics. playability for all these models are comparable, except for the 320MH, which isn't lacquered- i prefer this finish, you might not like the 'bare' feel...
take the cue from Marty Friedman- while waiting for his signature series to be completed, he chose to have a 320 as his main guitar- no fancy cosmetics, just something that works... i'm not saying the 520/ 720 are lousy either, if looks are important to you, then choose accordingly.
my friend if you are gonna change the pickups to EMGs, it doesn't matter which guitar you get- they will all sound alike.