Depends on where you are. Aside from my annual cross country road trip, I've been quite pleased with T-Mobile. I was very displeased with the amount of GPRS and/or EDGE-only coverage they had and the number of places I went that had zero T-Mobile coverage whatsoever. (You can voice roam on Monthly4G, but no data roaming!)
But as I said, when in town, which I am the vast majority of the time, T-Mobile is about twice as fast as at&t's HSPA network here, and only marginally slower than my SO's SGS3 on at&t LTE. at&t HSPA is not that good where I live. It'll do about 6-7Mbps, which is fine, but not as good. You should see even better performance out of a Nexus 4 on T-Mobile since it supports DC-HSPA.
Also, you can get T-Mo codes from a site like callingmart for 5-10% off and no tax. If you top up directly, you pay the full $30 plus taxes based on your zip code.
I may switch over to Straight Talk at some point, but only because 100 minutes a month is proving to be too little for me. T'would be nice if I could get another 200-250 for $10.--
It's wierdo, not weirdo. Yes, I know that's not the 'proper' spelling of the similar english language word.