Think about what behaviors you want to incentivize in your users. Is your biggest hurdle going to be building the expert side of your marketplace, or getting the best people deeply engaged? I'll give you my own experience from the perspective of a service provider having participated in these types of marketplaces at the two extremes. One is pure % of transaction, one is pure recurring subscription fee.
For the pure % of transaction, it didn't take much convincing for me to join their network. There was very little barrier there. But I'm finding that platform to be my lowest priority. If I'm slow or bored or I notice something particularly interesting come across, I'll pursue the project. And then I'll still do whatever it takes to exceed expectations as a matter of professionalism. But the % (which is significant in this case -- more than generic marketplace I think) significantly impacts my interest in working on their platform, and I am significantly less engaged in that platform. I wonder if there is a bit of reverse incentive here that might eventually degrade the quality of the service provider network. As the best service providers get busy and start getting referrals and other opportunities to work outside the platform (without fees), they are going to be a lot less inclined to participate in the commission-based platform. Which maybe means you end up with more participation from people who are just starting their practices, or who are just not getting a lot of work for some reason.
For the subscription fee, it was harder to convince me to join. I ended up taking a significantly discounted rate to try it out. But now that I'm in and its working, they've become a high priority. I monitor their clients' needs regularly, and I respond instantly to opportunities. I figure my investment is fixed, so I might as well get as much out of it as possible. I also feel vested in their success, because the more work they can bring onto their platform, the better it is for me and the better the return on my subscription fee.
Not sure if it makes sense from the business' perspective, but from the service provider perspective, I've often thought it would be good if someone offered options. E.g. let me join for free and pay a high % transaction fee to try it out. If I like it and want to work a lot on your platform, offer options to commit to a monthly subscription fee in exchanged for reduced transaction fee.