It's a matter of personal taste. I prefer having the sidebar rather than having to scroll up to the top of the screen all the time. The travel menu on mobile is all messed up and weird, I haven't been able to get routefinder to work at all. With a new char on colonies and zero previous experience with the island, that is a bit frustrating.
Then again others may have trouble with the sidebar, or not care about routefinder, and prefer the more streamlined feel of the mobile site. It definitely has its good points, they just don't outweigh the bad for me. I'd prefer to use the main site.