I own and regularly use both a PAPS X and a PAPS v4. They have different switches but I really wouldn't want to say one is better than the other, They are just different. The X is 23mm which is an issue for some and an advantage for others. Both have the GPin which is very convenient. If you are looking to pull a lot of current through ultra sub-ohm builds, that (GPin) may not be the best setup. The PAPS X is also available in a single tube size option, GP calls it the GP X. If you are only, ever, going to use one size battery that makes more sense.
As time has gone on and I spend more time using regulated mods I have sold off or given away a lot of mechanical mods (10+). Many of them a lot more expensive and a lot more "la de da" than anything GP makes. My PAPS X and my v4 are the only mechs I own and think for 99% of typical vapors they represent to best balance of quality, customer service and performance. GP service is amazing and the owners are accessible on your choice of social media or forums. If you beat the crap put of your mod and it's starting to look shabby, you can send it back to them. They will refinish it, give its the once over twice and send it back to you for less than a few bottles of premium juice.
If I had to choose I would go with the v4. I'd get the silver plated contacts upgrade because I'm lazy and don't keep up my contact maintenance like I should.