The bottom line is neither keeper should be a guaranteed starter for a team like Everton. If Howard's contract is up this summer, as I believe it is, we should be looking for a keeper of clear No. 1 quality to bring in. Ignoring that though I think there are certainly more positives to be found in starting Robles over Howard.
For starters, Robles should still have his best keeping years ahead of him, where as Howard is obviously past it.
Secondly, when given a run of games last season we all saw a level of consistency and assuredness develop in his game, while the only thing consistently seen in Howard's performances over the last 18 months is a tendency towards shambolic keeping. I'd much rather have Robles' consistent 6/10 level of keeping over Howard's 3-4/10 with the odd 7/10 performance poping up.
Hopefully by next season though we'll have gotten the check book out and will have a 8/10 quality keeper to call our own