Some here touched on this, but one of the issues to consider is CDMA VS GSM technology.
Straight Talk uses ATT towers and technology and thus is a GSM based phone with SIM card and all that goes with it. Thus, you can use ANY unlocked or ATT GSM based phone on ST. ATT has a very good network, and it is much larger than Sprint (VM)'s core network in North America. With either carrier you are not going to be able to roam, so with ST you can't just hop on a T-Mobile tower to roam, and with VM you can't just hop on a Verizon tower. Pre-pay phones here in the U.S. all work that way. No roaming.
Verizon and Sprint (VM) are CDMA technology. This stands for Code Division Multiple Access. It is only common to North America. You can google it and research for yourself. There is NO SIM card on these phones. The programming is done internally via keypad or OTA -over the air programming.
GSM is the European standard, stands for Global System for Mobile, and the SIM card governs the phone's programming. The SIM is the key and you can swap SIM cards at your will from phone to phone, as long as the phone is either unlocked or set-up for the carrier you are on.
This being said, then you look at plans and value for your dollar.
VM is pretty darn good at even the $35 Beyond Talk of DATA is your gig.
You get gigs of data and 300 anytime minutes. That's a bargain IMO.
The phone choices are good, and the plans offer some very nice options. Sure, you can't switch phones without either calling or going online to make the switch, but is that really important? The VM (Sprint) core network without roaming is not as large as ATT, but it's awesome in the areas that it is in. Compare ATT (without roaming) to Sprint (without roaming) and see if the areas YOU are going to be in work for your carriers.
Both carriers are going to be strong in some areas, weak or uncovered in other areas of the US.
IF you are going to travel to Canada, or overseas, an UNLOCKED GSM phone is a better choice since you can buy a foreign SIM, pop in into your unlocked GSM phone and be off and running with a foreign number faster than you can say "Salut mon ami!". None of the US Prepay carriers are going to work once you step foot out of the country. Again, they do not roam, period. You can't program a Virgin Mobile Phone to work on Rodgers Network in Canada or an Orange Network in Europe or any of the Non-US carrier networks, but you CAN do this with a GSM phone that is UNLOCKED.
What's important to you? Talking? Texting? Data? All three? Both plans have advantages and dis-advantages.
I happen to like VM very much. I was with T-Mobile for 10 years and paid through the nose. Now I pay a very small cost and have been very happy with VM.
Good luck with your choice.