I'm not sure that I specified this, but I want something without a contract. Also, I decided it would be easier to just buy from Craigslist. Here are the costs of various phones in my area:
iPhone 3GS
iPhone 4
iPhone 4S
Nokia Lumia 710
Nexus S
Galaxy Nexus
So you're right, they are slightly cheaper, but I would have to look at various costs with different phone services.
I just went through this process myself. I had been with Verizon for nearly a decade, but only had 1 smartphone - a 1st generation DROID. I read that Verizon is going to be capping data in the next year, so I decided to look into off-brand carriers. They offer no-contract, lower monthly fees for access, and - depending on your area - just as good coverage.
What I found is that most of the minor providers buy wholesale bandwidth from Sprint. Not all, but nearly all. MetroPCS and Virgin, I think, are the only ones who maintain their own network. There are, I suppose, advantages and disadvantages to that.
The company I went with offers VERY affordable no-contract packages, and doesn't penalize you for going over your package limit. In fact, it's not really fair to call it a limit, since they simply bump you up into the next tier if you go over your package cap. If I think I'm going to use 1000 texts in a month, but actually use 2000, they will simply add $3 to my bill. With data, it's basically $21/GB. With Verizon, I was paying $40 for 'unlimited', but that's on its way out.
The other thing to consider is many off-brand carriers don't subsidize the cost of the handset (smartphone) which is why they're so much cheaper. I picked up a used Samsung Nexus S 4G for $90 off Craigslist and my total monthly bill is about $45. Compare that to the $95
every month!! I was letting Verizon extract from me (sans Vaseline!)
As for the specific handsets you've linked, I would narrow the list immediately to the:
iPhone 4s [if you want iOS, but forget the 4 and lower]
Lumia [if you want a Windows phone]
Galaxy Nexus [forget the Nexus S - far lower processing power]
One thing to consider with the off-brand carriers, though, is that they may not support the device you want. That's a big deal.
I would consider first what carrier is best for you, and THEN go about finding a compatible device.
The iPhones are not able to be run on Sprint's network through off-brand carriers as far as I know, so take care on that point.
If I were to go through this process again, I would try to go with the Samsung Galaxy Nexus or Samsung Galaxy s2 - those were both above my budget at the time. But my carrier is great, and I would definitely choose them again.
EDIT:
just checked Sprint's coverage for Raleigh (all your links pointed there) and they've got WiMAX, which could be considered 3.25G service. It's not as robust as LTE - which is like 3.5G, but it's far faster than 3.0G. Like, things will load in half a second that via 3G would take 2-5 seconds. The processor also affects this, which is why I advise going with the Galaxy Nexus - dual cores, 2x the RAM, and a faster GPU. No brainer.