I think it would be better to buy Warband for you because Bannerlords gonna come out as a early acces game so it will have a lot of bugs,some contents will be missing etc.Also Warband is not a bad game you can play it for weeks before getting bored and while you're playing it Bannerlord will get patched,polished and also you can save money for Bannerlord while playing Warband until Bannerlord comes out of early acces.You can learn core mechanics of the game by playing Warband so Bannerlord would be more enjoyable since you know things about it's core and lore(Warband has some lore about the Bannerlord since it's 200 hundred years after Bannerlords events).Also if you get bored from the Warband you can always download mods and play multiplayer.