You don't really want any headroom at all for cider, especially when primary is finished. When primary is going fast you get a bit of froth, nothing like beer. Once primary is finished you want to keep headspace as low as possible, oxidation is the main cause of problems with cider. Glass is better, but plastic is ok for a few months. After that you run the risk of too much oxidation. Since glass carboys should last forever, I think it is worth the extra expense.