I'm only in the early stages of the game having just bought it this week and have a question - is alien improvement dependent on the speed I improve? I'm asking, because I have a heap of research done and can improve my weapons / armour, but don't have a lot of money. If I upgrade, will the aliens also become more challenging? Or is it just sequencial - the further I get into the game, the tougher things will gradually get?
Aside from leader aliens in UFO's which change after completing story objectives. ( Outsiders - > Sectoid commander - > Etheral )
Every other alien in the game is based on how many months have passed. Aliens get maxed out around month 6 i think.
So if you rush to plasma weapons before month 3 hits you will have a great advantage over low level enemies for the next few months.