You'll get a lot of opinions on this. If I had to choose, I'd give a slight edge to the eastern route. Mostly because there is an extra day at sea. I also like the ports better. I do like Grand Cayman, but Key West and Cozumel are just so-so to me. I also find it hard to drag myself out of bed on the first morning in Key West, after travelling across the country and the inevitable late night out on the first night of the cruise.
I've done 2 western and 5 eastern, mostly because the eastern has been offered longer. When it comes right down to it, I'd take the one whose dates worked best for me, regarless of whether it was going east or west. If the date wasn't a consideration, I'd go with the eastern. Either way, I don't think there's a bad choice.