Touché
I guess if I shop vigilantly for prepaid plans and don't mind managing several different plans depending on each users' needs, I would probably always pay less with prepaid. I was thinking simplistically that at some point, the cost of adding lines to postpaid would cost less than individual prepaid contracts (e.g. $90 + 3*$10 costs less than $30*5). But I probably simplified too much in my mind.
I did look at both Virgin Mobile and T-mobile after your question, and no, they don't have family prepaid plans. I also compared their post-paid family plan with their prepaid plans.
While you can eek out a small savings with the 5 phone post paid unlimited everything family plan ($265/month + ~16% tax = $307), that's because it competes with their $70/month prepaid plan ($350 + 0 tax = $350). Compared to the $60/month plan ($300) the prepaid wins (you can save another 3-5% with discounts from some retailers). But when compared to Virgin Mobile or Boost Mobile unlimited everything plan ($55/month), prepaid is the clear winner ($275). Boost Mobile even has the shrinking payments which drops the monthly cost to $40/month after 18 months.
Compared to AT&T unlimited everything family plan ($520/month + ~16% tax = $603/month
LOL Wut?), well, uh, prepaid wins. AT&T post paid loses even compared to their own Go Phone unlimited everything + 5GB data prepaid plan ($100/month + 0 tax = $500/month
LOL Wut?).
I understand wanting to pay it all at once, and I'm sure you can somehow with prepaid, but the premium for post paid plans are too steep even with the family plans to justify the convenience and contract.