IMO just buy to whever makes you feel is fun to play. Your matches are rigged to where you lose and win "X" while you play. Your skill level isn't a very big factor. 

This is honestly a tough choice. I think it's less a choice of which to buy, but more a choice of which to buy first. Orion is a fun and unique mech to play. Works well with missile weapons, pods, and sniper weapons. At the same time, the pilot makes pods work with almost any kind of mech (and of course any of the barrage weapons).

By the way, in my personal experience, spending acoins on the epic pilots has always been worth it. Legendary pilots are a different story. If you're patient, you can wait for crate rush events that give legendary pilots for cheaper or after achieving some objectives. That's how I've got all my current legendary pilots, and saved lots of acoins.