You've had that Jag for a Long time man...and in my experience, any guitar that keeps you satisfied for that long, needs to be kept forever...even if sometimes you're not digging it completely, or are using something else more. Clearly the Jag had exactly what you wanted for a long long time, and that means something. You might just be bored with it right now, or be doing something suited better to the 333...But eventually you'll need that Jag again, and it's way better to have it stashed in a case somewhere, than in someone elses hands. Ya know?
So I vote, Keep the Jag.
An SG isn't really the best backup for a Semi-hollow anyways, totally different sounds. If anything just score an older Sheraton, or Rivera as a backup to the 333, and have a true "backup".
An if you're bored with the Jag, why not try something new with it? Make it into a true Jag, single coils and all. Don't let it be a "backup" to the Humbucker loaded Gibson...Make it something different, so you can find uses for it that the Gibson can't cover....You could even rout it for a tradition Jag trem, and get all those groovy picking behind the bridge sounds, and a bit of waggle. I find having a few very different guitars much more useful live, than having very similar ones. Definitely makes switching guitars make alot more sense.
