I use both.. takes me maybe 30 secs to chock the wheels as Im unhooking and leveling. Then add the X chock for stabilization, and good to go. I do this whether I am on a level surface, or uneven.. doesn't matter.
I suspect the standard x chocks (for narrow axles), are probably alot more capable of keeping the trailer from rolling. I have to use the extended version and I can see that not being as solid/safe as a good ole wheel chock. Not to mention that I've read of situations (even on our website here), where the trailer rolled when using just X chocks. Was it because the tires cooled down and they weren't locked in? Who knows... fact it is rolled, and had the wheels been chocked with standard chocks, it would not have (or shouldn't have).
Mike