​They post first class so all depends on royal mail. Iv had them come next day sometimes 2-4 days

​The main differences are story and pokemon, theres not much difference in core gameplay. This is a good price just depends how much they like it

Alpha Sapphire and Omega Ruby are remakes of the 2001/2002 Ruby & Sapphire games, but with a lot of changes. Graphically they are now the same as X & Y but the story is completely different and there's a lot more to do.
As said above, it all depends on how much they like Pokemon, but its a great game for £23.

My 14yr old son who is pokemon mad says not its not worth getting this as well x

It is the exact same game, the only difference is that there are some Pokemon on one game that aren't on the other and vice versa. If he has a second 3DS from a sibling/family member they will be able to trade those Pokemon and battle against each other, but if there is no second 3DS available then it really is completely pointless. Even so, 20 quid to get a handful of extra in-game characters is not worth it imo, only if he had siblings also interested would it be worth getting one each