I work at Walmart and sell Straight Talk phones all day. So I can answer any question you want.
The simple fact is this. PrePaid plans maybe cheaper per month, but your costs are much higher in other places. Take for example the fact that the Android phones they have go for $150-$250. No price breaks, no insurance. So yeah you may save $35 a month, but you have to fork over the massive amounts of cash for the phone outright. Not to mention the fact that the Android phones they do have are about 2 years old already and no carrier even has them anymore, which is why they are on a pre-paid plan. Let us not forget that if something were to happen to the phone, you would then have to fork over that same amount of cash, again, to get the same phone. So what seems to be a pretty nice thing for $45 a month, turns out to be just about as much as you would pay on a plan.
That is of course if you drop the phone. Yet again this also comes down to how the service will be for you as well. Straight Talk bounces off the T-Mobile towers. In addition they are only 3G not 4G. So your speeds will be lower, your coverage might not be as good and you currently have. So if you do decide to get ST, I would watch yourself. While you can get a return on the phone cost at Walmart, the cards can not be refunded at any point.
Which reminds me, when you buy the phone, it comes with no service at all. Meaning that in addition to the $150-$250 you spend on your Android phone, you will have to add in the additional $45 right away as well. That way when you activate it, you can actually use it.
Dont really know what else I can tell ya. Prepaid does have its advantages, but it also comes at a heafty price.