OP, the speedtest.net screenshot you posted sums up my experience of having the 4S with Sprint perfectly.
Once I had the iPad3 on Verizon, and saw what LTE was like, especially on a network like Verizon's, I knew I would switch as soon as possible.
Now that it's happened, I couldn't be happier! Two days later, and I still keep getting pleasantly surprised by how fast the data connection is (to be fair, there's good Verizon LTE coverage in the DC metro area).
Now, I know that Sprint is going to be switching on LTE networks across the country soon, but I'd rather put my money towards what works well now, rather than gamble on Sprint getting its act together while giving them money in the meantime for subpar service.