I'm not here to prove why a certain version is better than the other version since these 2 are quite different from each other. This is my opinion.
1. Which version takes more skill?
Since 1.8 and 1.9 are both very different from each other and require different tactics, you can't really say which version takes more skill.
With 1.8, you can get combos with the usage of eggs, snowballs, and a fishing rod. Since there is no attack time, you can spam click your mouse, and combo a player like crazy (with the correct setup of course). Although PvP'ing on 1.8 is pretty easy, let's not forget about hotkeying. Every time you don't get a combo, you swap to your fishing rod to try to combo the other player again and again.
While in 1.9, you can get combos as well, but it's all about the way you move and with the correct timing. If you don't time your hits correctly, or move a certain way, the player can hit you as well and end your combo. Also, the way to start a combo is way different from both versions. On 1.9 there is almost no hotkeying regarding PvP.
As a 1.8 player, I find it hard to adapt to the playstyle and tactics of 1.9, 1.9 is a whole new playstyle than 1.8 which requires a lot of practice to get used to.
2. Why you think PvPers hate the opposite version?
I'm mostly certain this is because of the way to PvP in both different versions. Because 1.8 is more action-based and quite fast. 1.9 is slower and requires patience and a lot of practice for good combos.
It depends on the playstyle that all the players like. Patience can be good for certain people, while fast-paced PvP'ing is more a thing for me.
3. And all other supportive reasons you can think of why 1.9 is better. Prove me wrong
Every version has it's up and downsides regarding PvP'ing. While I absolutely love the regeneration on 1.9, the timing of hits and creating combos on 1.9 is way harder than it is on 1.8.