Best place to buy an iphone 5S?

I want to buy an iPhone 5S. I don't know weather to buy it from the store or buy it from Amazon. In the store it will cost me around €680, but from Amazon it will probably cost me €600 for the phone, but I'm not sure on the V.A.T included price fro when it arrives. I'm not sure if the price would be the same in the store and online from Amazon.

Here's the phone from the network I'm considering going with:
http://www.three.ie/online/shop/ProductDetail.aspx?pid=2010&p=voice&tariffType=prepay&step=5&src=p&edt=0&itemId=0&dch=0

Here's the Amazon Link:
http://www.amazon.co.uk/...00FBTZBDQ/

Iphone experts? Come hither?

When I got my iphone i synced it to my desktop and added all the music i wanted to on there with no problem. I recently just got a new laptop and I would prefer to make that computer my main one and I want to use it to house all of my music instead of my old one. I tried to use my new computer to add music onto my phone, but it wouldnt let me because its still synced to my old computer. So when I try to sync my music with my new computer it asks if I want to erase this phone and sync it with my new itunes library. It says it will replace the contents of my iphone with the contents of my new itunes library. So i'm not sure what that means. I'm honestly okay with it deleting all of the current music on my phone, so I can start fresh. But I don't want it to delete everything on my phone? I'm only okay with it deleting the music i have on my phone. What will happen if I agree to erase and sync my phone?

What's going on with my Iphone5?

I need to so something about my phone. Yesterday it went from 39 to 1 then when I plugged it in it went to 25 I unplugged it and it went to 38. Then it went from 85 to 75 in a few seconds. Then when it got to 71 it immediatly dropped to 67. Can someone Please tell me what I need to do to fix this?

What are the main improvements of each new iPhone?

Could you just tell me one or two of the new features that each new iPhone (e.g. 3, 4, 4s, 5…) introduced?

Thanks in advance

Added (1). More specifically, I'm looking for unique selling points for each (or a few) of the models - a little more detail than 'better camera' would be really appreciated, e.g. 8MP camera.

My iphone got stolem, and i have Findmyiphone app but what do I do when I locate it?

My phone and credit card where just stolen last night and I dint think about the "Find my Iphone" app until 3-4 hours later but by then it was offline, I know someone stole it because when i called it they returned my house call but only let it ring once,
So what should I do when I locate it next time it goes online? Should I call the cops to come with me? Or what can I do to get it?
I'm thinking about going with some friends and forcing them to give it back but at the same time why do that if i can just take a cop but then again I know you can't search someone's house without a search warrant… But could they search the person if its in the streets?