I have had sprint for 4 years and currently have 7 phones on my account. Then I personally have ATT strictly for the Iphone4. Come December Sprint will carry the iphone and I will be going back to their account. As far as quality, data wasnt quite as fast as verizon or ATT, but coverage and reliability is by far better. in the 3 years i was using them i lost signal twice that i can remember. and I traveled ALOT for work. Sprint always worked for me and never dropped calls. ATT on the other hand is the exact opposite. I have a work issued BB Bold nad that ALWAYS lost signal same as my IPHONE.
My suggestion is go Sprint. and if you want an Iphone, see if you can wait until Dec or use a prepay for a bit. then in Dec you can get the Iphone cheap with a new contract with them.
As far as the throttling. This was expected. Where as Verizon invested in coverage, ATT invested in Servers. Iphone stayed with ATT because they were the only company that can handle the data exchange that Iphone require. This is also why it is being released to all 4 major networks now that they are expanding to break up the data between companies.
BTW I was contracted for ATT for 18 months and worked out of their ATT Corporate building so im not really Bias. Just giving you experience from within,