MetroPCS vs Straight Talk
MetroPCS offers unlimited talk-text-web plans starting at $40/month. If you have a compatible 4G phone, this price includes 250MB/month of 4g data usage. This includes all taxes and fees.
Straight talk offers a similar plan for $45 / month, which may cost about $47/month after tax. Data is limited to about 2GB/month although Straight Talk does not acknowledge this publicly, they simply warn and terminate customers for 'abuse'. (Buying a year of service at a time reduces this price to about $43/month; smaller discounts are available for 3 or 6 months.)
 Service Availability
MetroPCS, like other carriers, offers excellent service within most large cities in the US. However MetroPCS service lacks 4G coverage (which generally means only 2g is available for data) in Oregon, Washington state, and many suburban areas or small cities. In many rural areas, Metro provides no data or voice service.
Straight talk simply resells AT&T service. As such they offer service in a great many places, including much of the rural USA and even substantial portions of Alaska. (Incidentally, MetroPCS doesn't allow calls to Alaska unless you route your call through a Google Voice account or pay extra for 'long distance')
Tested download speeds on MetroPCS tend to be in the 2Mbps range on LTE (which is actually half the speed of fast non-LTE data services and a quarter the speed of the most expensive 4g services). Tested speeds using straight talk (which does not provide LTE) are similar to MetroPCS's LTE, but available in many more places.
 Phone Availability
MetroPCS is not compatible with SIM cards and also may not provide full compatibility with flashed phones from other CDMA-type service providers. MetroPCS's best phones would be considered 'mid range' rather than high-end by other carriers' standards. Android 4.4.3 is currently the highest version of the Android OS offered.
Straight Talk is compatible with any unlocked GSM phone, including Google's Galaxy Nexus (which can easily be upgraded to the brand new Android 4.1 OS) and the upcoming Nexus 4.
Both carriers offer suitable choices for those who want an affordable low-end smartphone or a "dumb" phone.