Members will give you more exp and will be harder because there are more components of the floors e.g. herblore and better food allowing giving you more potential.
Members is tough I once got 100 dung on an account. I would suggest learning the tricks and trade about dung and how it works. Yes, members have P2p skills and involves more complexity but with a lot more reward! Best of luck!
Yeah, there is a big difference between f2p dung, and p2p dung which is why a lot of low level combat accounts start to dung in f2p before moving over to p2p. Also if you ever feel like you need help with dungeoneering just check my sig