Really hard to have a objective answer to this...
The most important point is that Global is online but London/Revival is single player only wich make a HUGE difference for a lot of people.
Global : - pros : online, a lot more content (SH, Abyss and the whole Tokyo zone). I don t add 2nd attack "extension" as it s the most painfull thing we have ever seen in this game.
-cons : You ll have to endure a crazy shit load of bugs all day long. Server is a joke, game will crash, DC etc...They will never be fixed.
You ll have to pay for a lot of aspects that make the game fun. Paying to safely have a chance to remove your mods of a weapon is the worst thing for me. You ll have a tiny inventory and stash.
For the vanilla Hellgate, feel free to try if you like the core gameplay and get used to it, you ll have hours of (solo) fun. The Revival mods highly enhanced the experience and the anticipated Nagahaku mod will bring buckets of fresh air.