personal preference. I prefer the fat tires for my commute, 35 years of motorcycle riding and I feel more secure in traffic on a fat bike when doing 30+ mph.That & the price of fat tires is why I chose not to go fat. They are more comfortable, but there can be
handling issues as well.
on my weekend rides which are 99% on bike paths and trails, the bike I use has 700c x 38c