Wow that's a hard decision! I'm gonna have to say go with the Balenciaga. I used to be a Chloe girl and just recently came over to the bbag dark side. Although I really really like the Betty style, I have to agree with oogiewoogie that the Day looks cleaner. I could forsee getting the Betty zipper pulls caught on things, etc.
Trust me, you'll love Balenciaga leather!