both offer Bring Your Own Device plans that are cheaper than the subsidized plans for the usual players. T-Mobile just announced a fully unlimited $70/month plan.
The unfortunate thing about these plans is they don't have 4G LTE, as T-Mobile is only up to what I'd call 3.5G with their HSPA+. It's ok, but not as good as LTE.
That said, I'm seriously considering one of these once my Verizon contract is up. I'm looking at getting the Nexus 4 which is only $300 unlocked. It's a fantastic deal for one of the best spec'd phones out there.