Should I train chain for him or just stick to leather?

I'd stick to leather since leather expert eliminates the recovery penalty allowing to act faster than with chain.

I always pick chain for clerics.....in all the M&Ms, i like the extra defense....even if i am a bit slower, in M&M7 the cleric is little more than a M*A*S*H unit so i go for survivability...

Here's the deal, you have to keep you Cleric/Priest alive, so you should strive for expert chain and master shield. With those two skills your priest won't ordinarily take much damage, assuming you have her equipped with good armor (Golden is best) and a decent shield. Note that Priests cannot master any of the armor skills except shield and cannot wear plate at all.

It's actually easy to get Golden chain early in game by harvesting Erudine nuggets and visiting the armor maker in Erathia. The best/easiest place to get the nuggets is Tatalia.

Later on place a nice armor increasing enchantment on the shield and armor and you are set.

Chainmail only offers a little more AC than leather, nothing else. The difference is low in the beginning and even the best chain only grants +36 AC while the best leather grants +24 AC. Considering that you also get AC bonuses from from shields, gauntlets, helmets, boots, cloaks, spells ... there'll only be a few percent AC difference.
And AC only helps versus physical damage but not versus the many other damage types most monsters deal.

Faster recovery on the other hand will let your cleric act faster in combat, the extra actions can be used for attacking or casting healing or offensive spells more often during the battle.

And I would say - that depends on your team. If you have e.g. knight, thief, cleric and mage (the default set as far as I remember) it's good to pick chain mail for cleric as you will definitely use leather for both thief and mage. Otherwise you may find a really cool chain mail that nobody can use and that would be a shame wouldn't it? Not to mention that finding three decent leathers may prove to be a bit tricky.

On the other hand if you replace your thief by archer (as I always do) you already have someone to use the chain mail. In this case I would stick to the leather armor - as kmonster mentioned: the difference in AC is not that big and your cleric will move faster.