It depends on where you live (coverage). T-Mobile's 3G/4G HSPA is way less susceptible to buffering because of the higher max speed. But if you can't get HSPA or have really congested towers it will probably still buffer on T-Mobile. That said, in Los Angeles the coverage is supposed to be pretty good with a high density of cell towers.. but 3G coverage is currently less than that of super slow GSM/EDGE because T-Mobile chose to pinch every penny. AWS panels only on one sector or no 3G at all in areas they don't consider worth the investment just yet.
Happy T-Mobile Customer since '09
Content AT&T Customer since '12
Sprint? Accolades for most improved still doesn't mean good enough.