Join Date: Apr 2005
Location: SW Washington, USA
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I'll take a shot at it.
It really boils down to which shifters you're using.
STX was Shimano's "good" quality component line in the mid-90s. It was superseded when they went to the XTR-XT-LX designations about 10 years ago. I believe you're right, LX was designed to be a notch higher on the quality scale..... but in practice, I can't see that you'd notice the difference. Especially if you're working with used parts (are they used?) -- then the best part is probably the one with the least wear.
I don't know the Suntour X1 specifically, but Suntour and Shimano use(d) different transfer ratios on their derailleurs. If you've got indexed shifters, then you'll want to use a derailleur to match. Maybe you're lucky enough to have Suntour shifters in your parts box -- then use the Suntour derailleur. Personally, i love old Suntour stuff. Smooth as butter, and easily serviceable.
Without knowing which shifters you've got, i'd assume they're Shimano indexed. Then i'd go with the LX derailleur. It's a standard on many touring and mountain bikes, which pretty much indicates the quality.
If you have friction shifters, you can use any brand derailleur matched with them.
As i said, i don't think you'd notice any difference between these three derailleurs, both in shift quality and longevity. These are all good mid-to-upper range components, and should handle the task equally well. Things that might tip your decision are weight, dust boots, smooth running pulleys, and -- if they're used -- any wear.