difference between sharing files using wifi direct and using wifi hotspot

One basic difference in usage is that if you use access point approach, any other access point can not be used same time, meaning that if user had existing connection through some access point, it will be terminated.

Also if there are apps looking for internet connectivity, they might start pushing data as soon as you make a access point connection to the other device, which might pose some security issues (in case the other device has some sniffers ready for sensitive data etc.)

With WiFi direct, there are no such things as access points, Basically you find a WiFi direct device, and start connection to it. With first creation of the WiFi Direct Group, there is dialog shows for accepting incoming connections (so its always shown in the party we are making the connection to).

Then when the connection succeeds, one of then devices are chosen to be Group Owner, and any other devices wanting to join the group, should then make connections to the group owner only.

With WiFi Direct API, you can also use CreateGroup, to pre-select one device to be group owner, This option would also create a Access point with random password.Thus if you want to go with Accesspoint option, you could use this feature for it easily.

For the easier/faster questions, I would not have any good answers, I would suppose they probably are around the same.