If you are happy with sprint then stick with them. I would say that verizon stays in business for 2 huge reasons.
1) Their speeds are generally better, and with their 4G network rolling out faster than sprint and on 700mhz vs 2500mhz they have nowhere to go but up with service.
2)Many people stick to verizon because many of their friends have verizon. You see if your friends all have verizon, even if you have sprint, sure it won't cost you a penny to talk to them on their cell phone, but they will all use their minutes to talk to you, this was a huge reason why my sister and many others online seem to refuse to leave verizon.
Also verizon isn't that much more expensive for most plans, I think it's about the same in cost +/- $10-$20 a month if you don't have a family plan with data on every line.
Also I am just curious as to which plans you were looking at that made verizon $70 more a month, because I imagine you'd be looking at the Sprint Everything Data Family for $129.99 a month, or on verizon the Nationwide Talk and Text family for $99.99+ $30/line for data, so if you are both smartphone users then it only comes out to $160 a month which is only $30 more, not 70. The only possible cause I can imagine for needing a more expensive verizon plan is if you or your fiance talk for hours during the day to people not on the verizon network.
All the same, there are reasons to go to either one, but if sprint is cheaper for you and you are fine with their internet speeds in your area then they sound like the better choice, I much prefer the phones they get anyway.