Upload & Sell: On
The photo of the newer Castel-XQ II makes it look like it has a gear track underneath, and has a drive knob to move the upper rail along in the base. It also looks like there is a clamp down knob that will lock the movable rail in place. those are both important features, especially the good clamp down method. One thing that is missing it seems is a quick release to allow fast movement of the rail along in the base. The quick release is an additional feature of the Really Right Stuff rail that is handy if you are changing your set up a lot.
I have never seen or used one of the two direction QXD clamps that Novoflex shows on the rail. Since it is a screw clamp, it likely will fit a wide variety of Arch Swiss plates and brackets. But in looking at it, there are only three of the corner things that clamp the plate, which is a kind of different than a normal screw or lever clamp. It may work fine, but for me I would want to play with one before ordering. Obviously that was the question of your post, so lets hope someone comes through with experience.
The movable rail in the NovoFlex Castel-XQ II rail system also shows both 1/4 inch and 3/8 inch threads as extra places to mount things. That means if you did need a different clamp, or wanted to mount something else on there, you have a way to do it.
There are no markings on the bottom dovetail of the the Really Right Stuff rail. If you need a calibrated scale for indication and need that for over the maximum 103mm travel of the RRS main slide, you will need to work some other system out. If you mounted the rail in a clamp that was about 50mm long, and the clamp did not have the relief areas in in like most RRS clamps do, the safety screws on the bottom of the RRS dovetail would contact the clamp in a way that the total travel was about 70mm. You could then measure that and use it as a known movement to either add or not add that fixed amount. You also may be able to add a scale or ruler to things in some way.
The Novoflex Castel-XQ II relies on the one scale to be read by both the movable items. In its case the upper clamp slider is still fairly close to the scale, so estimating is likely reasonably accurate. The Really right stuff rail system has the upper scale further away from the clamp below, but in evaluating it and how accurate the estimating could be, it looks like you could easily get it within plus or minus 1 mm, so that method may work out for your needs based on what you descibed. With a normal clamp that would add about 110 mm of additional movement, which helps, but is still short of what the Novoflex offers.
The RRS slide has very finely engraved markings every milimeter for the scales, and there is one on each side of the top for convience. The lead screw moves the stage about 1.25mm per turn, and using that it is easy to move the stage very smoothly, and estimate position on the scale. There is a knob on each end of the RRS slide, and one end has a larger diameter because it does not have a clearence issue like the other end does. The knob is not marked with any scale to allow rotation movement to give calibrated fine movement, but markings could likely be added. I have done focus stacking sequences with the RRS rail with good results, but have not yet tried one that needed high precision movement.
I understand the issues with high prices for Really Right Stuff items in Europe, which becomes a significant issue in choosing what you want. I do see that RRS will ship internationally, but duties and taxes and shipping may make a local purchase at a store over there a better deal. Good luck with your decisions.