Im planning on registering my saas product as a company and I am wondering whats the best option their is for solo-founder. Is Stripe Atlas the best option in terms of easiness to setup?
You can incorporate in Canada. It's fast, it's inexpensive and it makes your bookkeeping, tax return work easier. I think Atlas was mainly started to enable companies in countries where stripe can't make payouts yet. If you live in a country like Canada there's no need for it.
Shopify (also a Canadian company) has a nice guide for it: https://www.shopify.ca/blog/7116328-the-ultimate-guide-on-business-incorporation-in-canada
Don't make that decision based on Stripes shiny UI.
What gains do you see by doing that?
In my opinion, it really depends what you're selling, where you are selling to and what you're trying to establish. Going through the likes of Shopify means when scaling you're stuck as you haven't got a the ease of other solutions, you ideally want someone to take care of the all the complexities that occur around billing for your solutions like the handling of tax, vat and compliance in other countries.
A colleague of mine looked into these different options it's worth reading -> http://bit.ly/2Us1OWM