I vote for gold for main currency with emeralds being rewards of quests with which you can buy special stuff.
Assuming we'd have to pay with gold instead of coins, there would be even less world-guild-pvp (not random pvp whenever you encounter another player) as you usually need money to quickly pick up new armor and weapons near the battleground to be able to get back into the fight quickly. For example: If you died and respawned at a graveyard, you'd have to run back to your town/plot/residence to get some gear or gold. So the fights would be over as soon as a part of one guild was killed, because they'd need quite a while to get back to the fight, whereas in the meantime their guildmates get killed.
Even more so if the opposing guild kills a few people and gets their gold (that some of them might have carried). Would they really risk to stay longer and fight more people, if they have the gathered gold in their inventory?
I don't think so. So even the winning guild would retreat just to not give the other guild the chance of winning their gold back, leading to incredible short-lasting fights.
It also encourages hiding behind your safe walls even more. Leaving the house at a time when everyone is online with the intentions of buying items comes close to commiting suicide as loads of people will camp the market (unless a HG directly leads to it or it is PVP-OFF).
On the other hand having gold as currency instead of coins sounds like a great idea to me lorewise.
Trading with gold seems to be a lot more realistic than paying with "coins" that can't be taken from you. Back in the days you did pay with a bag of physical gold coins. And Herocraft is a lot more into the "lore-direction", so why not go for another approach to get the "Hero"-feel.
In DG for example: I've bought a Nexus Plot in the 2nd or 3rd week after the map start and have never had to worry about being stolen from (besides exploiters), because it was impenetrable. Nobody could take my coins away either. All I could lose when exploring was a set of armor, my tools, food and the items I found during my journey.
And where was the RP-feel? Nonexistent!
I believe gold as currency would change that drastically. It would encourage player interaction, having allies, knowing who your enemies are and not just bluntly sitting in your DG Plot all day (what I was doing for 3+ months of DG).
And it would enhance the thief class. Honestly I've never played a thief, but I've never seen the point in it, unless you would've encountered a player that unknowingly carried his stacks of valuables in his inventory.
And as a quick edit: Why is everyone against change? "If it isn't broken, don't fix it" <- I get that, but why do you fight trying to improve it? Yes, it is balanced, it was convenient. Too convenient in my eyes...
And where would Herocraft be without steady improvement, testing and trying new concepts? Not in the state we all love it...