If I was going to go with breastplate armor for me, and mage armor on the eidolon, our ACs would be relatively decent. But, if I have to spend 10 minutes time taking off my armor so I don't blow the 25% miss chance it forces a change in plans. Yes, I want the celestial armor, but that will be about a year or two away. And the armor mastery route is two feats, which would also be a long time away.
If the answer is yes, I could take longer buff spells for the summoner side, start with +2AC for me, and cast those buffs ti-chi style.
If the answer is no, I start limited to chain shirt, and go for faster attack spells on the summoner side.
Since everyone is going to insist on me taking haste as soon as it is available, and I wouldn't have the celestial armor by then, I would have to do the light armor only at 4th level anyways.
Role for the character: Smash mouth battle cleric. Work hard, party hard, bar brawling good ole boy that wanders the natural world, even though he and his companion are not a part of nature. Commanded by his god to fix the minor problems as he finds them. Never comfortable in high society or formal settings, but will visit a dragon or satyr to chat and learn it's stories and tales. (Oh, defender of dragons too. Will work against anyone attempting to challenge the grand beasts.)
Side note: Dragons are nature's safety valve for ambition. It is easy to accumulate power. It is easy to think you are invincible. Eventually most fall into the ego trap of "I can kill a dragon" and will try. The dragons weed out the foolish ones that think they can wield great power without consequences. There are more princes buried in dragon outhouses than castle cemeteries.
Next question: How many hit points for the eidolon? At first level, and for his normal progression too? They generally start hit point starved, and this gestalt thing throws a kink in the process too.