Other than the fork, not a huge difference in specification on the Giant / Cube. Mongoose is a completetly diffent bike – great if you want to launch yourself off stuff and go fast(er) downhill and not worried about how long it will take to ride back up.
Of the 2 I’d probably opt for the Giant and get my upgrade purse ready, as within about 6 months / 12 months of riding most of the drivetrain / wheels stuff will be done for if you ride 2 times a week or more.
Dont forget also that once you obviously get the MTB bug, decide to sell all your road bikes, kit, gear, book, mag subs and cancel your club membership, you might have a little upgrade pot to play with 😀