I don't think prospect quality is judged mainly by NHL-readiness, so that's not why I voted Allen. But he has proven himself and then some in the AHL and shown enough at the NHL level that he's free of the classic goalie doubts for me. Now it's just a matter of how good of an NHLer he becomes and how quickly he does it.
For a while now, I've only seen one really notable flaw in Allen, and that's swimming caused by a tiny bit of over-commitment. And even that's not that bad. He's big, calm, fast, well-positioned, is a great puck-handler, has a terrific glove, and is better at rebound control than any goalie the Blues have had for many years.
I think his ability with the puck is of particular note. It's not just raw skill, it's his ability to read plays. He seems very aware of what's going on and is going to happen. When was the last time an otherwise NHL-caliber goalie played the puck so well and didn't stick?
I love Fabbri's potential, and I think he's a fine choice for #2, but, right now, Allen's floor seems higher and his ceiling is at least as high.