Which Baritone? Ibanez RGIB6 or PRS SE Mike Mushok?

Either are good choices but I find that low-mid tier ibbys are very much "try before you buy" guitars, much more so than most SE PRS. I would personally avoid the RGIB6 unless the seller has a good return policy (in case you end up not liking the guitar). The only other quibble would be that if you dislike the EMGs it's a bit more work to swap them out for passives or other brands of actives like fishmans.

The PRS mushok is a great guitar imo, I love mine. It's got a beefier neck than the ibby, where the mushok feels like a chunky D shape, and the RGIB6 is more of a wizard shape neck.
The mushok tends to sound a bit darker/growlier with the stock passives. The stock passives are actually pretty good imo, but they're easy to swap out/sell if you don't end up digging them. The hardware is good quality but nothing amazing. The stock bridge is pretty comfy imo though.
I've got a pretty bare bones demo vid of the stock pickups if you're interested.

 

I'd go with the Mushok, purely because I used to have one and I almost regret selling it. The Mushok has better features, and overall better build quality. The stock passives in the Mushok are fine as well, I used to have a DiMarzio D-Activator the bridge because it lacked a bit of clarity, but that is entirely dependant of your rig. The shoddy QC from the Ibanez Iron Label series put me off massively from that side of the equation.

 

Low G is absolutely doable, even on shorter scales. You probably don't need the 28 but it would be nice

I've not played a Mushok for longer than a few minutes so I can't really comment on that one, but a friend had a RGIB6 that I spent some time with and it was very nice.
He had it tuned down to E (to mirror his bandmate who was using an 8 string) and it performed perfectly well. It had been set up by a pro though, for what its worth.

Good luck!

 

I’d pick the RGIB for that thin neck but, i haven’t tried either.

 

How many frets do you need.

 

Recently got a SE Custom 24. It’s an absurdly nice guitar, hits way above its price range. If I was shopping for a bari 6, I’d be looking hard at the PRS. Really interested in the newer 7 string.

 

The Ibanez has a higher chance to be a lemon, and the PRS needs some work before it's a great guitar (nut and pickups).

I'd say get the Mushok. Even though you need to get upgrades, it'll still be a great guitar.