I've started using treadmill, should i try to get as much outdoor running in or it doesn't matter?

I prefer running outside. Lots of people on this app gave me some great tips and recommendations to run outside in the winter but i just cant afford the shoes right now. I mean i could but i have a free gym membership and i need a new car and im hoping to eventually save for a house.

Does treadmill work my muscles differently? Should i try to run outside as much as possible still? I mostly stopped because of icy sidewalks.