I only know of one Wesley/Westley myself, so I wouldn't say that it's common.
I prefer
Westley, because I like the tiny bit of 't' that comes out when I say it. (So, therefore, yes, I say them differently, but only slightly so.)
I like the nickname West, too. I would choose
Westley, nn West. Good idea! It's nice to have a nickname and a first name that you love.