I'm also a fan of chocolate supreme. With regards to whey vs casein, I believe casein protein is a much slower acting protein. I would always use whey during the day and drink a casein shake before bed for blood amino acid production throughout the night.
I would suggest NOT getting the cookies and creme flavor as i could never get rid of powder chunks in the shake.
If you have any tendencies to be lactose intolerant make sure you go for the ISOLATED whey.
Personally, I like to go plain. You can blend it in with anything and you won't have to worry about a chocolate spinach smoothie tasting disgusting (which, it will).
On it's own with water, it just tastes like a glass of skim milk.