I'm pretty ignorant about current offerings in derailleur systems, but a couple years ago I bought a cheap REI bike that came with an xram 3.0 rear derailleur, and grip shifters. I've absolutely loved it; found it rapid and completely reliable with my wide-range gearing. I ride on pavement and don't bash it against anything. Perhaps that's why I've found it so satisfactory.
They don't offer that one any more, and I'm building up an old frame and want to have grip shifters. They now offer X.4, X.5, and so on. The satisfactory performance of the 3.0 I have leaves me to suspect I'd be happy with the cheap x.4, but I don't know what grip shifter would work with it. They've introduced terms like "compatible with ES systems", or "1:1 ratio". I know enough to realize I've got to get the appropriate number of speeds on the shifter (probably 8), but am worried about getting a mismatch in terms of compatibility. If I bought an x.4 or x.5, can I make a mistake in choosing the shifters? Any great advantage in the more expensive ones, in view of the gentle treatment they'll get from me?