One has an "X" on it.
Coming from a 66 year old mid teens handicap weekend hack, the Pro V1 is a little softer and a great ball, but for me the Pro V1x is favored. I get a little more distance and a lot straighter shots with it. Seems to have a higher trajectory off the tee and also mid irons. I'm not much for hitting a draw but when I can pull it off the Pro V1 is easier to draw. Best advice is to buy a sleeve of both and on a slow day take a couple shots and see what suits you best.

Plenty of info on the differences of the balls under the "Balls" category on the website. Too much info to repeat again on the forum.

I always found the X to be a tad harder and got more distance. The pro v1 always seems softer to putt with in my opinion.

I found that the left dash X is even better than the 1x. I feel it more forgiving for my game. I’m a 58 year old, 10-11 handicap. I still see good control around the greens and great stopping ability.

The x feels a bit firmer, and flies a bit higher. Both are great balls.

Not trying to be rude, but there are hundreds of articles online comparing the top. Just google it, or look at Titleist’s website