is that worse or better than only just appearing on the scene?
One guy is relatively young, has talent to burn, a good technique, finally got his chance in FC cricket and has taken it with both hands, has international experience (and success), but his sample size is small.
The other guy is less talented, been around for ages, has experience on his side but has had some very ordinary patches, has never truly dominated, and is unlikely to stay in the side all that long. He's vanilla, but you know what you're getting, and he'll be a decent stop-gap measure before somebody better comes along.
Personally, I'd give the younger, more talented guy a go at this point. See what he's made of. If we were genuine contenders but were missing a solid opener for an upcoming year of tough cricket, then I'd go with Cowan. But we're rebuilding, and Warner has put in better performances (but over a much shorter period of time). If we take a risk and it fails, well, our team is pretty damn rubbish right now anyway. If it succeeds, we've taken our first step towards rebuilding.
Basically, playing Ed Cowan doesn't move us forward as a team, and right now, we need to be moving forward. That doesn't mean gifting games to players who don't deserve it (e.g. Starc shouldn't have been selected as there were clearly better options), but when all else is equal, you go for the higher risk, higher reward option.

