It depends on how much you currently make. When we joined up, it was made very clear that a 90/10 contract was the breakeven point with what we were making under adsense because our CPM and CTR were very high already. Joining up to make the same money didn't make sense to us because we had no use for the additional features offered by Fullscreen. So we had to ask for something better and they were able to offer us a contract that gave us exactly what we wanted. It's obviously different for every channel. There's a lot of people who make more money with a 70/30 contract, so it's possible you'll do better with them. If you're concerned about that, ask them about it and they'll be able to tell you after seeing a few screenshots of your adsense numbers for the past few months.
No matter what network you decide to go with, make sure you getting something in return if you have to give something up. If you are making less money because of their percentage you pay out, make sure they give you something useful for that-whether it's some sort of promotion, tool, or even having a partner manager call you every 2 weeks to work on strategy. If a network can't offer you anything in return, find one that will-or just stay on your own under adsense until you get the contract you want.