IPhone 7 Plus or iPhone 8 Plus?

I'm due for an upgrade for my iPhone 6. I like the Plus models and I think I want to switch to those, I'm just not sure which series. There are slight differences, but none seem to really justify the price jump for me for the 8. But I also don't want to be stuck with a slow phone in a year because I'll be 2 models behind.

How do I use my verizon sim card in a sprint iphone?

I ordered a used iphone on amazon and I clearly picked "verizon" as my carrier because I have verizon, but they sent me a sprint iphone instead by accident (I think). I read the invoice and it said verizon, so I assumed it would work anyway. I tried putting my sim card in and it said "invalid sim." I have been looking all over the internet on how to unlock my iphone, but a lot of sources say that it's impossible to use verizon on a sprint phone.

How should I go about this? Should I just return it? Or attempt to unlock it somehow?

Question for the App Store on iPhone 6s Plus?

So before I use to use a friends iCloud or Apple ID whatever. But when we were both using it, it showed a little cloud simbol basically meaning that you've downloaded it already. I made my own Apple ID or iCloud and now it doesn't show the little cloud thing, when I download the app and in a few days take it off it says " get " not the little cloud thing. Which for me is good. But my question is, the little cloud won't show anymore right? Since I'm the only one using it?

How do you replace an iPhone? Is it difficult?

I have an iPhone 6s refurb cause the first one was faulty… Anyways to keep things short I think the battery is on its way out… I'm not willing to pay$80+ for an iPhone battery if I can get a kit and do it myself from Amazon… Is it hard to do? Easy to mess up?