Mining gold, what familiar should I be using to maximize my xp per hour:

 

Pack yak or Lava Titan

 

(I know I currently can't summon either, but I'm collecting charms to get the desired level)

Lava titan is superior, even at 99.

Lava titan is superior, even at 99.

Any thoughts about xp per hour difference between using one of both?

It seems the Pack Yak is generally better xp, although it's a minimal difference. However, with a Pack Yak, you need to use w77 instead of w84. There are too many familiars in w84, so it makes it hard to use a BoB. The conditions are much better in w77.

Lava titan is superior, even at 99.

Any thoughts about xp per hour difference between using one of both?

 

I was getting around 3k~ more an hour with a lava titan. I used a yak for 3-4 hours, just to compare them, I don't even bank gold anymore.

 

I've seen fairly large logs on the RSOF that would also seem to indicate a titan is better.

I did some tests of my own and found that yak was slightly faster, although popular opinion seems to indicate otherwise. I suspect that those who favour the titan are not particularly adept at spotting their familiars. Of course, the more people there are, the harder it is the spot your familiar, and so the smaller the advantage the yak has over the titan.

If you drop/alch the bars after you superheat them, may as well use a titan. If you bank, use yak.

For the small difference between the two (haven't had pack yak experience yet, however), I would go with a titan. Reason being, if it doesn't have a huge advantage over a lava titan (which it sounds like it doesn't), its not worth the extra effort. I am admittedly lazy, so the less that is required of me, the better.

 

Having said that, yaks do have an advantage if you want to bank your gold. The more you can hold per trip likely will give you more exp/hour. If you are powermining it or superheating gold, a lava titan would be your friend.