Both the Princess and Celebrity have the same stops. Both start from Seattle. So it seems to me I'm really just shopping the ship itself. They both even cost about the same (close enough to not be an issue)
I've read of some ships have piano bars where the crowd gets into it and sings along. That sounds fun.
And how is the Celebrity dress code in actuality? No jeans to dinner, I get that. But after dinner, can I put some jeans on and explore the ship?