Tough choice. Amazon PC download for this game on the PC is the best deal I've seen anywhere for now. $40 and you get the Prof. Genki stuff. It's a Steamworks game anyways so I'm not sure exactly that's going to work.
Buying direct from Steam is $50 for the game and Genki stuff and physical retail here is also $50 and I'm not even sure if you get the Genki stuff, either.
Otherwise I'm seriously considering buying direct from THQ. For $50 you get free shipping on the physical version otherwise you can do download of the game, the Genki stuff, AND...here's the big difference: "A Season Pass that includes three downloadable mission packs " <- If that's three $10 a pop DLCs you can figure out the value there pretty fast. THQ says it's around $20 or so worth of value but even at that it's still potentially pretty compelling.
Well, while I do want the free DLC kicker from the pre-order bonus, I just love the feeling of having a physical PC DVD-ROM in my hands, not just a digital download. Sure, I would need Steam to play the game, anyway, but having the DVD on hand would really cut down on download times, and get you going right out of the box right away. Plus, if I build a new PC (especially with Windows 8 coming out, I usually build a new PC when Microsoft releases a new version of Windows), I'd like to have a DVD so that I can move SRTT to that new rig. We'll see how things go.