I tune my guitar fairly low, drop A, and I really dig the gnarly low sound it gives the instrument but intonation is always an issue on a "standard" guitar. So I'm wondering, will a baritone guitar such as the PRS Mike Mushok model retain that same 7-string low chugging gnarly sound or will it just sound like really thick strings?
I guess it would keep that chuggy sound you want. I mean, that's pretty much the whole point of baritone guitars, low tunings, isn't it?

Check and see if your local guitar shops have any baritones in stock, have a go on one.
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A baritone would be perfect for that
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A baritone will have a tighter low-end than a shorter-scale 7-string. But with the right amp getting chug is not a problem unless you really want the sloppy loose low-end that stoner bands go for.
Why not get a 7 string?
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Why not a 7-string you ask? Well, I really am looking into one like a COW 7 from Jackson because I really want to stay in my established Jackson family. But I dunno. That's a lot of strings. Just a tad intimidating.
TS, just go out and try some 7's, for your sake. If you don't like them, too many strings, then whatever, go for your baritone. But, if you like playing 7's then that opens up probably 15 times more guitars that you can get compared to baritones. It makes scales etc., easier too with that 7th string as opposd to a 6 sting bari.
This post may contain my opinion and/or inaccurate information.

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Quote by AWACS
TS, just go out and try some 7's, for your sake.

As a baritone guitar owner I second this motion. Unless you are just expanding a collection guitars or you want to play Type O Negative songs a baritone is not the most useful guitar to have around. Then again, I really, really love fingerpicking old Carter family songs on a guitar tuned to B…
Baritones do have a slightly brighter, tighter tone to them. That's the nature of having a longer scale length. If you were to get a short scale guitar and tune that the same, it has a much softer and warmer tone.

If standard scale lengths give you the tone and feel you like then go for a 7 string. Unless you do lots of thumb-over playing you won't notice much of a difference moving from 6 to 7 strings. You'll just get that added low B, or A in your case.
Yo OP if you get a 7 you can be in drop A and have a standard tuned guitar underneath...
A 7-string and baritone are different. Don't confuse the two. A 7-string will sound like a 6-string tuned to standard with an extra string while a baritone will uniformly be lower. It's usually a different timbre and the chord shapes will be different. If you want a baritone, get a baritione. A 7-string won't fill its role. There's a reason baritones still exist after 7-strings were mass-produced.

But to answer your question, it will sound different due to the scale length. It will be tighter. That might be the opposite of what you're looking for though. The only way to know is by trying them out.