If you go with B series mobo make sure they have enough PCI lanes for 1070 and m.2 at full speed.
There's nothing to make sure. All B150 boards have 8 lanes in addition to the 16 lanes you get from the CPU. This means in addition to full bandwidth (
not the same as speed) for the main PCIe slot and the M.2 slot, you have 4 lanes to use for other PCIe add-on cards such as WLAN. Usually the x4 PCIe slot will operate in x1 mode when the M.2 slot is populated, becaus there are one or more PCIe x1 slots that need to have bandwidth as well.
The only other variable with regard to bandwidth is whether the M.2 slot accepts M-key devices. B-key devices can only use two PCIe lanes. I'm pretty sure, though, that all LGA1151 boards that support M.2 are M-key compatible.
However, the OP did not say he's getting an M.2 SSD. Most likely he'll be using a standard 2.5" form factor SATA 6gb/s SSD.