To many variables......you need to explain where, how, what conditions, what snow conditions etc....etc ....etc you ride in for us to give you our opinions.
It's only 4" more on the ground than the 155". I chose the 155" because in Western Washington we have wet heavy snow and I boondock more than hill climb. Those are the 2 main reasons. I went from a 156" with 2" paddles to a 155" with 2.4" paddles so much more traction. New tracks are way better design and ofcourse so are the sleds. I figured my 02 took me everywhere I wanted to go with the 156", and a few mods so I imagined the new D8 and 155" would do more than I wanted. If I lived in Idaho or Montana (anywhere there is deep DRY snow, and real hill climbs) I would have went 163". I also like having to work a little to make it to the top, makes riding more enjoyable for me. If it ever turns out that the 155 just wont get me enough traction I guess I will add more power. The 163's also started arriving almost a month before the 155's but there wasn't really any snow here anyway. I have to friends with M7, one with 153 and one with 162. I noticed the 162 was more difficult to manuever and didn't give me more that a couple sled lengths on hill climbs.
So things like snow conditions (dry, wet, deep, etc), riding areas (terrain), your riding style, your size, should be on your list of pros and cons to help make your choice. Burning up hyfax not a big deal, just get hyperfax. Hope that helps you.