Why there is a diffrence in the price of ipad in apple store and other retails?

Other retailers can set their own prices.

As to which is better it will depend upon what you are planning on using the iPad for and how you will be using it e.g. will you be spending a lot of time holding it whilst using it, would a smaller screen be sufficient (if for reading ebooks and browsing/email the Mini might suffice), or do you need a larger screen ?

Is the device being sold by Amazon or by an amazon reseller?

Authorized Apple resellers will usually have the same price as Apple since Apple sets that price for Authorized sellers. They can offer promotions and bundles though. But non-Authorized sellers can set whatever price they wish since they are not selling Apple retail-new stock items.

Amazon itself is an authorized seller for ipads but they also offer their own refurbished devices as well. There are no Apple authorized Amazon resellers though.

If you are checking the correct Apple site for your country, and at an actual Apple store, there should be no price difference.

It's not usually that large a price difference. You are sure it's the same model and spec e.g. same capacity and both wifi-only or wifi-plus-cellular (cellular is usually £100 more) ?

The Mini 4 has been out since September 2015, it's possible that third-party sellers (it's not Amazon selling them) have decided to reduce the price