There is no difference between IB and Scottrade Pro execution. My Scottrade Pro account uses IB's platform, with orders routed using IB's SMART routing. I also have the option to choose all of the other exchanges for routing.
I've been very happy with the execution. A specific example that comes to mind is when I was first getting used to the platform, I had a position that was showing as a short spread, with negative prices. I was confused by the negative pricing, so when I went to sell the position I entered the order completely backwards. I wound up placing an order to buy more of the position with a limit price higher than the ask. IB executed the order right away, but the price was actually lower than the ask that was displayed, just a couple of cents from the mid. I was quite impressed with that.
As for mobile, I've never heard of the Handy Trader App. I have downloaded and used the IB TWS mobile app with my Scottrade Pro account. Everything worked fine.
So in summary, a Scottrade Pro account is held at IB, and I have all the same execution, routing, and platforms as a regular IB customer. The only 2 differences are the cheaper commission structure, and the fact that i can call up the Active Trader group at Scottrade for customer service and support.
The 50 cent per contract commission structure is a huge advantage, especially if Kim keeps finding cheap calendar spreads on low price stocks