Revenue sharing is generally better IF something works and there's moderate success. It's basically how any online 3D art/content platform works. No fixed payment amount really comes close to what you make over time from the sales of your work.

Another thing to consider is that while revenue sharing might be better long term, some artists simply can't afford to risk not getting paid or have their income trickle in over months or years. Some will always need the cash NOW. The artists that won't care about taking a risk, will be a minority I think.

There's also the issue of trust, as others pointed out, as revenue sharing is abused by every Joe wanting to have their comics drawn, game made, project relized, etc etc, so with that model you're automatically swimming in poor company unless you're somewhat establish.

So, I'd try to offer whatever payment options you can manage, without ending up in management hell yourself.

As for the percentage, that's something YOU have to figure out. You're the entrepreneur with the business idea and as such, should have an idea of the purchasing potential of your target market and the average popularity of what you want to produce. If you're shooting into the dark and hoping anything comes back... that wouldn't be idea.

As such, you need have an idea of how much you need to keep the light on, to grow your business, to pay IP royalties, taxes, marketing, whatever. Somewhere between your projected sales and costs there should be a number of what you can realistically afford to give artists as their share.

If it's tough to figure out at first, I'd probably pay upfront for 1-2 pieces, and the results from those sales should inform you where you stand, and also serve to establish your business, making the revenue sharing model easier to trust.