The "unlimited" data on that plan is only 500MB of HSPA speeds. The 30-dollar T-mobile plan only starts throttling after 5 GB of data.

Once you go over 100 minutes, it simply starts charging from whatever balance you have on your account. You'll get a recorded message warning you, then I think it's $0.10/minute thereafter.

I usually stay under 100 minutes or go just up to the edge. If I'm at home and expect to be on hold or talk for a long time, I'll just use Google Voice from my computer, which is better than holding a phone anyway.

I look at it this way:

Plan #1: $30 for 100 minutes, with $0.10 a minute afterwards.

Plan #2: $40 for unlimited minutes with MetroPCS.

So, if I manage to talk less than 100min + ($50 - $40)/$0.10 = 200 minutes per month, then I end up spending less than I would on the MetroPCS plan.