In case this is useful for anyone out there, I did some comparisons based on my needs (3 iPhones). Note: I'm currently on the family plan and two of our iPhones are grandfathered with unlimited data. In the below list, I didn't break out all of the specific costs. The final amounts are the total sums based on 3 phones. If you have less/more than 3 smartphones, the scales could be tipped differently in terms of which carrier and specific plan makes the most sense.
1) AT&T - Existing family plan:
550 mins w/Rollover ($60, lines 1-2 incl, each add'l line $10)
Unlimited family messaging ($30)
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300MB each phone (no tethering): $160
3GB each phone (no tethering): $190
5GB each phone (with tethering): $250
Adding an iPad w/built-in cellular requires a separate plan for the iPad (e.g., $30 for 3GB)
With the above plans, you can mix/match the data amount per phone. So I could keep my unlimited data plan, and switch my wife's plan to a 5GB w/tethering plan, for example.
Because we don't use a ton of voice minutes, and only need 3 lines, I selected the cheapest voice minute plan.
Limitations: No support for tethering or FaceTime.
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2) AT&T's new shared data plans
Unlimited talk & text, tethering & FaceTime included
4GB = $190
6GB = $195
10GB = $210
* Can add iPad w/cellular for $10 more
Data overage charge: $15 for each add'l GB
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3) Verizon's shared data plan
Unlimited talk & text, tethering included
4GB = $190
6GB = $200
10GB = $220
* Can add iPad w/cellular for $10 more
Data overage charge: $15 for each add'l GB
Note: they no longer offer a non-shared-data family plan
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4) Sprint's Everything Data Family plan
1500 mins ($130, lines 1-2 included, each add'l line $20)
Unlimited data but no tethering: $150
* Note: considered to have the slowest data speeds
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5) T-Mobile
No, they don't have the iPhone (yet), but just in case they get it, I figured I'd take a look. Well, I either did something wrong, their website is screwed up, or they're doing some false advertising somewhere. This is what I *thought* I could get:
1000 mins (lines 1-2 incl, each add'l line $5)
2GB, no tether = $125
5GB w/tether = $155
10GB w/tether = $215
Instead, when I simulated buying 3 Android phones with the 5GB plan ($155), the final total rang out to $190.
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Based on the above, the current AT&T Family plan I'm on, supplemented with a separate 3GB iPad month-to-month plan, ends up at $220, which is the same price I'd pay with AT&T's new shared data plan w/10GB. My current plan gives me two unlimited data iPhones, one 3GB data iPhone, and one 3GB iPad, whereas the 10GB shared plan obviously limits my data at 10GB total, but that would *probably* be enough.
Verizon works out to be $10 more for the 10GB plan.
If I thought I could live within 6GB total, I could save $15 with the new plans. The new plans would also allow me to tether with my iPhone (which I could see wanting to do on very rare occasions, but can live without it) and use FaceTime (which could be nice, but I can live without it, plus I could always use Skype).
I'd be worried about whether 6GB would be enough, though, because I use a lot of data, and my daughter uses a healthy amount of data (and I suspect her usage will increase). And while my wife uses very little on her iPhone, I'd worry that she'd use a lot more with her iPad.
The great irony is this: The new shared plans *encourage* you to tether, connect iPads, use FaceTime, etc. All of those things are more apt to then cause you to go *over* your GB limit, and thus get hit with overage charges.