If choose the chantry garden:

• Can I still grow herbs, and if so, how many?

Yes you still can grow herbs from the 2 pots. (1 in each pot)

What exactly is the "potential resource gain" and how frequent and useful is it (Do you get rare upgrade material or just commong things like iron)?

It's suppose to give gold from followers, but I have found no way to get that gold ...

• How frequently and in what amount do you get the money from tithes?

See last answer ...

If I choose the herb garden:

• How many herbs can I grow there?

You pass from 2 pots to 6 pots.

• Do they grow faster, or is there any other herb growing advantage compared to the chantry garden?

Not that I noticed...

• Does it offer any other advantages?

There is an achievement to gather 50 herbs in the herb garden.

Reference Link

I would go with the Herbs Garden, but you can still choose. Have fun!

If choose the chantry garden:

• Can I still grow herbs, and if so, how many?

Yes you still can grow herbs from the 2 pots. (1 in each pot)

What exactly is the "potential resource gain" and how frequent and useful is it (Do you get rare upgrade material or just commong things like iron)?

It's suppose to give gold from followers, but I have found no way to get that gold ...

• How frequently and in what amount do you get the money from tithes?

See last answer ...

If I choose the herb garden:

• How many herbs can I grow there?

You pass from 2 pots to 6 pots.

• Do they grow faster, or is there any other herb growing advantage compared to the chantry garden?

Not that I noticed...

• Does it offer any other advantages?

There is an achievement to gather 50 herbs in the herb garden in one playthrough.

-EDIT- It is not needed to have the Herbs garden for this achievement, but it is way easier with 6 pots.

Reference Link

I would go with the Herbs Garden, but you can still choose. Have fun!