I'm not sure if this is a good choice or not, but I'm thinking of maxing out sentinel recast but unsure what else I should put merits into for group 1. For group 2, I have 5/5 Chiv, and am thinking about 5/5 Guardian.
Shield Bash or Rampart are the typical group 1 options people pick after Sentinel. Both are useful in my opinion, but neither is game changing.
Chivalry is still useful outside of Abyssea and terrific when soloing, and Guardian goes well with your Sentinel merits, so all good. If you find yourself not using it, though, may want to switch most of Chivalry merits to Fealty, just for those situations where you'd like to be free from status ailments for a 30 sec breather, or when you know Mijin Gakure or Astral Flow may be coming soon, etc.
On the magic side of things, I'm thinking of 8/8 healing and enhancing. Is this a good plan?
Do you have other jobs you plan to play? Do they have magic skills? If PLD is absolutely the only job with magic, go with that plan, but unless your other job is WHM, they probably won't fit well with other jobs' needs.
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If I understand it correctly, full Divine Magic merits adds 2 points of damage to each proc of Enlight. That doesn't sound like much, but depending on how often you recast Enlight, the typical average damage figures hangs around 22-26 for most Lv.85 PLDs I've parsed. At an average of 20, 2 more points means a 10% increase--we love Brutal Earring for a measly 5% increase to melee damage, so 2 points there (for a 10% increase) is actually impressive. At 25 average Enlight damage, that drops to an 8% increase in damage, which is still pretty nice.
For Joyeuse + Shield users, Englight is a terrific way to tag on additional damage (which is good, even if it doesn't contribute to enmity). So while I personally wouldn't go with Divine magic merits, for those who do not have anything else to merit, it may be useful for bumping up one's damage a bit more. Keep in mind that the more often one recasts Enlight, the less of an upgrade Divine magic is. Same with increase in level and the increase in Divine magic skill it brings--this is worth less the better the player and higher the level goes.
As for Enhancing magic, each 10 skill equals 1 point of damage reduction from Phalanx; one would get either 1 or 2 points more damage reduction from full merits, depending on gear.
However, Phalanx is not doing any good unless one is getting damaged, and /NIN is still the standard SJ for PLD (at least, it is for me). I suppose it's nice for fighting things with spikes (and en-damage when PLD/WAR) that can't be dispelled, since additional effects are counted separately, but like most PLD merits, it won't make a huge difference.
Healing magic merits... Well. I'm just not a fan of it, I guess. If it affects spell interruption rate more (and more visibly), I may reconsider, but as it is, it really is for the "I've nothing else to merit!" situation.