I've done both, several times. In my opinion, I would never consider a one way cruise, unless I was going to invest at least another week for interior Alaska touring, meaning- excellent Denali park, take full advantage of being there, also a must for me is at least a full day in Seward with a a Kenai Fjords boat tour.
Celebrity is even less attractive on their one ways only due to no College Fjords, which if doing a one way, strongly consider including. Round trips offer plenty of excellent touring packed into 7 days. I disagree on the Seattle sailings, they just do way too much outside passage sailing for my preferences, Vancouver is superior. There can be more scenic sailing on round trips depending on the routing. Do your homework, consider ports time in ports, glacier, route, price. You'll quickly find out Alaska isn't a one time trip. Go with whats fits your priorities best.