Ok... I just did the whole deal a few hours ago.
I ended up buying an iPhone 4 off of CL for $80 and traded that in instead of my 5S. I traded in the 4S for $120 since it would have cost me another $80 to find a phone that I could trade in instead and the market is glutted with them and you really can't get a lot over $200 for them, so I didn't figure it was worth it. Maybe I could have made $50, but I just wanted to get this done. I also traded in a Blackberry Curve 8520. The other line didn't have an ETF, so I didn't trade in anything else.
I got 2 new 5Ss and 2 new 5Cs. I'll sell my 5S (which is unlocked) for at least $600 or so. I was in the store for about 1:45, which is not too bad.
Ah, I guess they caught on. Was there a reason you didn't get a cheaper phone from tmobile and use the unlocked 5s you already had?