We've got a mixture of slabs and gravel, hoffi, which gives a bit of variety but also means there is adequate drainage. We are higher than our next door neighbours and I would worry with all slabs that all the rainwater would just drain off under the fence. You need to leave drainage - plus also you will want some sort of drain yourselves so when you clean off the slabs (which will need doing from time to time) it goes somewhere, not sits around. We've planted some low-growing shrubs and bulbs in the gravel so there is some softening, and we have loads of stuff in pots. Easy maintenance!