A micro-ATX motherboard will have 4 expansion slots vs. the usual 7. Most of the time, we have plenty of unused slots, so a micro-ATX motherboard will be fine. You might even consider a mini-ITX motherboard with only one pcie-x16 slot.
Smaller motherboards will usually be a bit cheaper. I would not consider the power savings to be significant.
One advantage of a small motherboard is that you can fit it into a small case.
I went mATX motherboard because the price was too good to pass up. But while you can find mATX boards cheaper than ATX board that doesn't mean that all mATX boards are cheaper than ATX boards.
Power savings - depends entirely on the motherboards. Not necessarily better or worse mATX vs ATX. Regardless of which kind you get, if you're not using extra features enabled through other chips on the board, if it's a quality board you can shut those off in BIOS and save yourself some power. Turn them back on when you add, for example, expansion cards.
Expansion slots - yes you will have less. Do you need more?
Cramped space around the CPU heatsink - again depends on the layouts of the individual boards. Many mATX and ATX have similar distance between CPU and RAM, and CPU and the first PCIe slot.
BTW sounds like you've concluded going faster than 1333 MHz isn't necessary. I agree with that. But I also wanted to add that with an Intel i3 or i5 you cannot run RAM faster than 1333 MHz without OCing your CPU or making tradeoffs - e.g. turning on XMP with 1600 RAM gives a very mild OC but by default on most all boards turns off Turbo and power saving features like EIST. Seems like a lot of people flip the XMP switch without realizing the full impact.
also you'll wanna check numebr of memory slots on mATX bioards. sometimes they only have 2 and will max you at signifigantly less memory forcing earlier upgrades... but provided the board you're looking at has 4 and anouth pci slots for ya then there's no reason to not use one. the extra space will be mostly wasted but you can possible mod or use the space for more airflow