I've got a mutt bike that serves as my gravel/winter/light touring bike. It's components are a mix of Deore, LX and XT. The LX long-cage RD recently suffered some misfortune. I was able to straighten it enough to get functional but no where near perfect shifting. I can't find a NOS LX long cage anywhere. What I have found are the current production Deore and XT versions. The XT is roughly twice the price of the Deore but either is within my price range.

My question is, how do the current production Deore and XT 9sp RDs compare to each other and to an approximately 8 year old LX?

The bike isn't a lightweight (27 lb with cro-mo frame) and isn't used for anything competitive so a few grams doesn't make a difference, but smooth, reliable shifting and durability are considerations. I'm willing to spend the extra for the XT but only if there is a significant improvement in performance and/or reliability.

For a mutt bike like that I would just look for a used 9sp mountain RD on eBay. Looking at eBay history of sold listings associated with a search of "rear derailleur 9 speed" shows 5 that recently went for $10 or less including shipping.

I doubt that you will notice the difference in performance among the families. Reliability, slight possibility, but derailleurs generally last so long (unless they come to a bad end like yours did) that you will not notice.

Does it need to be NOS? If you put an ISO in the C&V forum, you should be able to find one pretty easily.

No, wouldn't have to be NOS, just in good condition. Our regional bike co-op just closed its doors, or I would have tried there first. I am a bit apprehensive about buying used parts online, sight unseen, as I've had a couple of good buys and a couple not so good.

well it's not completely sight unseen, assuming the seller is competent and has provided a number of quality pictures. And if they're competent and have a good track record as a seller, they're probably also honest (a few bad ratings will kill an eBay seller business). And if they're dishonest, you will probably be able to get satisfaction from eBay. And in the end, we're talking probably $20 or less at risk. And your time and potential aggravation of course, which you might value differently than I do. I'm super cheap and am willing to risk a lot of time/aggravation to save a few bucks :-)

Derailleurs dont have "speeds" they just shove the chain sideways from one cog to the other .

well a modern "11s" derailleur is probably not able to pass an old-school 5-speed chain, plus the last few (5-10?) years have seen a proliferation of cable pull ratios that have decreased compatibility, but yes, most "8-speed" and many "10-speed" derailleurs will work in a Nx9 drivetrain

[QUOTE=GravelMN;18867417

My question is, how do the current production Deore and XT 9sp RDs compare to each other and to an approximately 8 year old LX? .[/QUOTE]

I haven't tried current XT, but I have ridden Deore. It works as well or better than old LX. Which is to say it works. Once adjusted it's been perfect.

Yes, 10-speed road will work on a 9-speed MTB, but I doubt it will suit the purpose.

Shimano 10- and 11-speed MTB derailleurs use different pull rates. I suggest a new 9sp Deore (Shadow or non-Shadow), or an Alivio unit if you're trying to keep the cost down.

MTB derailleurs get used hard. New Deore and Alivio are not expensive and it beats buying someone else's problem.

Actually the cage width varies to suit the chain width. You can usually get by using a RD up or down one "speed" but if I'm replacing it, why not get one that was designed for the chain width I'll be using?

XT would be way overkill - what is the largest rear cog you are going to use?
A long cage Sora will work for most riding. $25
Shimano RD3500 Rear Derailleur No PCKG > Components > Drivetrain > Derailleurs | Jenson USA

If you need more capacity Deore is the best bang for the buck - $33
Shimano Deore M591 SGS Rear Derailleur > Components > Drivetrain > Derailleurs | Jenson USA

I would buy something new but lower end. Technology from the top groups trickles down over the years and i'm sure there was also some development since then.

I just bought a Claris RD for my beater road bike and i'm very surprised how good it performs, and it only cost 25 bucks brand new at my local shop...

My go to RD is the Shimano M592 - inexpensive, and is rated for a 36T rear cog if you want to go that big.