Hello PlaguesKill,
I just have a couple of questions that my help us give you some better advice on the trade and whether or not it's in your best interest.
Have you ever played a Fender short-scale before? If you have, what was it?
What year is your SG, what electronics are you using, and is it a Gibson or Epiphone?
What type of music are you playing?
For the first question, if you've played a Fender short-scale before, then you know what to expect. If not, so major differences would be the Mustang's neck compared to your SG. Your SG has a 24.75" scale and a 12" radius fretboard. The mustang has a 24" scale and 7.25" radius. The .75" difference in scale length may not be a huge concern, but the radius will be substantially different. This may or may not effect your play-style and is just something to think about. I would recommend going to a guitar shop and asking to play a Fender with a 7.25" radius neck, whether it be a vintage style Strat or Tele or even better a short-scale.
Second question, when it comes to your current guitar, Gibson's range from $650-$3000+, so knowing what your exact model is would help decide. I'm assuming your SG uses either humbuckers or P90's, which are typically much higher output than what a stock Mustang or this Mustang (with Strat pups) would output. If you're using humbuckers, expect significantly more "noise" from the Mustang while using distortion. Nothing that is unworkable, and I'm not saying at all that the Mustang can't handle distortion, but just expect more background noise from the Mustang and use distortion or overdrive in moderation. The clean tone from the Mustang will be typical Fender bell-like, the greatest clean tone of all time.
Third, when it comes to the style of music you're playing, the higher output pickups of the SG is great for screaming distortion or fuzz. The Mustang is great for singing clean tones or mild to medium overdrive. They are both terrific guitars in their own rights, and I think you'd be happy with either one. One thing to consider is what someone above has said, most people I've heard of love stock Fender Mustang's when they first get them, but eventually the fun of them wears off and you may want your SG back or just to modify the Mustang. In the case of a real 1964/65 Mustang, I would personally not mod that anymore than it already is, mainly because you can get a reissue Mustang and modify it more if that is what you want, and also because the real vintage Mustang is more valuable in a stock setup than a personified setup.
Good luck deciding!
