This is a good question, and one that can easily be answered with the following key:

If you are a...

- Hammerdin
- Smiter
- Holy Shock Zealot with Dream, but you want higher physical damage and want to max Conviction
- FoH'er and you max Conviction

... then use Combat GCs because not only do the skills get a better damage boost from being increased themselves, but you so indeed get a nice defensive and blocking bonus with Holy Shield, or in the Smiters' case, both.

If you are a...

- Holy Shock Zealot who uses Holy Shock and NOT Conviction with NO Dream
- A Sacrifice Paladin
- A pure physical Zealot
- A Dream Paladin who does NOT wan to max Conviction with hard points

... then use Offensive GCs. When you are saving points in Conviction for something else, such as using Dream, but wanting to max Zeal, Sacrifice, Holy Shield, Resist Lightning, and have points for all the prerequisites, then you may only want a few points in Conviction, so the +~9 from charms help you hit slvl 25 without hard points.

The build that gets the MOST use out of Offensive GCs are physical Zealots. This is because after maxing Sacrifice and Zeal, adding Combat GCs will only increase Zeal's damage by 6%, while each additional point in Fanaticism nets a 17% increase instead, with a comparable attack rating bonus as well as the IAS (though IAS will be negligibly increased at that level.)

Of course, I'd like to point out that in that case, maximum damage charms will get you more damage than the percent from Fanaticism OR Zeal.

In your case, as a Hammerdin, you will get a much bigger boost from Combat GCs, though your merc is something to consider. Of course, doing magic damage and having 95% of monsters susceptible to your attack, which should already be pushing 11k with NO GCs, increasing your own damage by very little just to also increase your merc's by very little isn't worth it compared to a huge boost you'll get from Combat points. You SHOULD get an increase of about 1k if not more with every 2 Combat GCs, while it would take at least 3-4 Offensive GCs to get the same hammer damage. Doing so would only get a few hundred points at best for your merc.

For a personal example, I use a Hammerdin with NO Combat GC's and 2 Offensive GCs just because they have a nice life bonus. My merc uses a Guillaume's, Obedience Cryptic Axe and Fortitude Boneweave.

His MAX damage after Battle Orders with my lvl 33 Concentration is 13,285. If I take away ONE Offensive GC, it goes down to a max of 13,211, a ~70 difference.

My hammer damage with lvl 33 Concentration is 9083 (I traded my torch. Shut up.) With one GC removed, it becomes 8887.

So for each Offensive GC, I get about 195 Hammer damage and merc gets about 70 average Jab damage.

Combat GCs are the way to go. Unless you meet the criterion above, I'd stick with Pcombs all day.