I think the lighter weight is better for slipcovers. If you're going to wash, it would need to be prewashed before making.
A liner can be made if the old fabric shows thru, but it'll cost more. The only way to know if it'll show thru or not, is to lay the fabric on the sofa and see.

Thanks for your response. I figured if I was fretting about it so much, I might as well get the 10oz. I'm stretching my budget as it is, so I wouldn't want to pay extra for a liner.
I really appreciate your input!

10oz will work fine, but just make sure to prewash first.