It would be completely up to you. It would be nice if you could learn both, but if you can't do both, in my opinion I would choose english.
I would probably choose english because I personally do show jumping and really enjoy it, but as the others said, it's easier to go from english to western than from western to english.
So I would suggest first mastering english, then you can maybe give western a try and you will find you can adjust more easily.
Let us know which you choose and how it goes. Good luck.