2/OTT Underground Wire/Cable Needed

I ran 2/0 to my garage last year inside grey conduit, just in case something went wrong, then I could pull it out of the conduit without having to dig up the cable. Cost me around $300 for 70' of it. I had someone tell me to go with aluminum, I believe if its buried it could be to code. Do yourself a favor if your burying it, put it inside a conduit.