I can't answer the avg income query, [and don't know how accurate a projection for 2 years from now would be, because it's tough to project for right now, given the many variables involved.]
I can toss some differences between expediting in a S/T and OTR as a company driver in a t/t, though. The S/T offers more flexibility in where to park [can't take a T/T to Cracker Barrel, or most movie theatres] and generally less 'dock time', and there's less equipment to deal with [no sliding tandems or airlines to hook & unhook]. The downside is that expedite is like an ambulance or taxi: you don't know where you're going next, or when, until someone needs you.
You will be an independent contractor in expediting, running your own business, responsible for your own 'bennies', but able to turn down offers that don't meet your needs. [Depending on the circs, there may be penalties or repercussions - but in theory, you can always say "no".]
A team in a T/T will have as many miles as they can run, every week, when running for T/L or LTL carrier - not sure about expediting, there aren't a lot of them here. The enthusiasm for working 70 hr weeks wears off after a bit, though.
All things considered, the difference is in how much work vs home [or just off] time you want to achieve, balanced by how much money you need to make.
