I'm going to be installing another service for my buddys rental, he may go with a 100 amp or 200 amp panel. Now I believe I'm going to use 4/0 aluminun to the meter I figure this will cover either 100 or 200 amp. Now from the meter to the panel box is around 25-30 feet. Should I continue to run 4/0 aluminum or should I use copper witch will be smaller and if so what size. I'm going to use anti oxidant on the aliminum. Just want to make sure the wire is sized correctly.

If you stay with #4/0 Al, you have 100 up to 200 amp covered. Problem is if a 100 amp panel is installed, the 4/0 will not fit in any of the input lugs.

Really need to decide which size panel will be used and then size the wire from there.

The difference in materials between 100 and 200 amp is significant and difficult to mismatch amperage ratings of wire and equipment.