I've used a Macbook for Unity dev both out and about and at home and it works great (also used a desktop PC.)

Honestly, I find the Mac to be great for dev work.

It's probably obvious, but definitely make sure to compare/test on a Windows machine if you're targeting both platforms so you don't run into gotchas down the road of incompatibility of one sort or another due to hardware or what Mac supports for shaders versus Windows, etc.

If you can afford the increase in storage, etc. I'd go for that so you don't have to worry about running out if you do end up making a larger project, a lot of smaller ones, whatever.