Neither of them are any stronger than the other. 8X has a little less loss but thats about it. If your concerned about where the cable will be, possible contact with sharp edges, hot metal, chaffing, etc. I would sleeve it all. Get some rubber hose from the local parts place and run the cable through that.
Probably either will work just fine. All cables have losses, the thing with that is the length of the cable. For shorter lengths, less than 100 feet ('fudge' and say for distances of less than 50 feet), there isn't much difference and probably wouldn't ever be recognized at HF. At VHF/UHF things are different, losses tend to accumulate faster there. Still, for mobile use and short runs, either will probably do just dandy. Stick with premium cable though!