You can play both, zero mission is gonna take you around 5 hours to complete, or less, fusion around 8. Both are amazing experiences!

Zero

It seems to be a tie between those two. I would be a fool to skip one of those two if they are as good as each other. I guess I will have to find time to play those two.

What about the length of the two games ? In hours ? Can't find a trustfull source on the net.

Zero mission is a bit better that fusion

Depends how you're getting them. Zero Mission is expensive as shit on an original cart (relative to other GBA games), especially if you look for CIB. Once Zero Mission hits the NA eshop it'll be a toss-up. They're both great games so it's all what you're looking for that determines which is better. Fusion is a better starting point though, it eases you into the series' mechanics better.

I'll play it on an emulator on my vita, I don't deal with the "get the cart at an aweful price" anymore regarding GBA games and such. 

That's pretty fair. I don't even want to collect GBA games so I'm holding out for the inevitable Wii U eshop port for Zero Mission in NA. I'm sure, as a collector and Metroid fan, that I'll end up picking it up physically one day, but not until I can justify blowing $60 on a four hour game. 

http://m.ebay.com/itm/181826122802

Just in case you were serious. 

It all depends on a number of things.  But there are two major variables:  1) do you want to 100% the game and 2) have you played them before.  The second is obviously a no for you.  But the first will be a major factor.  They can be anywhere from a two and a half hours to over six hours depending on the number of items you look for, your skill, and your playstyle.  This is going by my save files.  On each game I have two:  a faster one looking to just go through the story, and a slower one with a higher item percentage.  The time difference between the two is quite large relatively speaking.