Here are some differences between the two series.
Vikings and Vikings: Valhalla are set during the beginning and the end of the Viking age, they represent Norsemen as a Westerosi-like people, I don't consider Vikings and Vikings: Valhalla as a Game of Thrones-like series but as a 300-like series.
In The Last Kingdom, they represent Norsemen as Barbarian, pro-slavery, chaotic, and anti-Christians which in Vikings are almost like this, but the Norse religion is misportrayed for example they use Sorcerers instead of Shamans.
Vikings and Vikings: Valhalla are about the Viking age from the Norse point of view and The Last Kingdom is about the Norse invasion of England.
Vikings was the first series I saw about the Viking age and is one of my favorite series but I dislike The Last Kingdom because they misportray the Norsemen, and I like other multimedia content about the Norsemen like Assassin's Creed: Valhalla or The Northman.
What do you prefer? Vikings or The Last Kingdom? Let me know in the comments below.