IB has a formula for rebates, you can do quick math to see. I've done it for my trading and came up with a <1.0 multiplier that I use for commissions forecasting in my models. Basically, I consistently pay a fraction of the listed price. But you will need to do your own math on that. Depends on a few things.
When is comes to execution quality comparison - unfortunately, no one here will share this with you. Mostly because most people don't do such comparison and simply don't know, those who do - keep it to themselves. That kind of quality information is very hard to get and it can make or kill your strategy and that might be part of your edge.
Like someone else suggested - there are some edge case trading scenarios when it is indeed might be better to go with another broker. Maybe that's a case for you.
If is really anyone's guess what you gonna lose on executions quality. One penny difference might cost you the commissions benefit plus some. But if you end up trying - let us know how it goes.
Val
