There's a handy table to compare the difference chipsets on wikipedia:
http://www.tomshardware.com/answers/id-2990807/questions-b150-h170-chipset.html
Basically the H170 has 2 extra USB 3.0 ports, extra PCIe lanes and hardware RAID support... none of which are particularly useful for a standard office or gaming PC.
M.2 is for ultra high speed storage. The PCIe drives which actually make full use of the M.2 slots are pretty expensive at the moment and while they're theoretically much faster than standard SATA SSDs, gaming and office computers don't often generate the kind of workloads that allow those faster drives to stretch their legs (think of it like a high end sports car vs a standard car. If you only ever drive to work in rush hour, that sports car will make almost no difference to your commute times). If you intend to keep the computer for a long time and upgrade down the track, M.2 slots might start to make sense. But you can use an M.2 to PCIe adapter if you need one. There's no performance penalty, it just places the M.2 drive into a PCIe slot.