Maybe I'm not spoiled but I wouldn't ever pay to upgrade my SLT. It marks fish better than I need.
As for the degree, you will notice a difference between then. Don't get the 19 degree if you fish deep water. They will all work for shallow water but I would get the 19 if you do 20 feet and under. 9 degrees will only make a couple foot cone at that depth. I did the numbers for this awhile ago. I have a 12 and it works great, but I never fish more than 25 feet.