"Customer Service" is a moot point in answering this poll. Without decent service/coverage, the greatest god/goddess at the call center isn't going to solve your crappy end user phone experience.
Having said that, I'd go for whichever carrier doesn't drop Unlimited Data plans when the (yet to be released) '4G' iPhone arrives on the scene -- and provides good phone service.
I fear that with the increased speeds, the growth will be so substantial (everyone and their pet will have a smartphone, congesting local cell towers) that UL data will become a thing of the past, industry-wide. AT&T is showing no signs of reverting back to uncapped data plans.
But I don't know enough about the 4G (or whatever you want to call it) infrastructure that maybe UL data plans will be the norm once again.
I mean heck, how can Verizon and others show TV ads with folks streaming Netflix, or ESPN promoting their sports entertainment viewing by encouraging us to "call your local Telco Provider to watch it on your phone"? Overage charges would suck the lifeblood out of this marketing paradigm.
Then again, 10 or 15 (?) years from now, smartphone data plans may be an arcane product that was sold for about 20 or so years -- with [free] ubiquitous WiFi everywhere in place of it.