Basically, King's Bounty: Crossworlds is nothing more than an enhanced Armored Princess, two mini-campaigns and an editor.
The changes to Armored Princess in Crossworlds take form in Rise of the Orcs. That's the same campaign you played in Armored Princess but with a greater emphasis on Orcs, some new units for most/all races, new items, new spells and a new mechanism called Adrenaline. Think of it as Armored Princess 1.5 if you wish. I would probably suggest that if you've not played Armored Princess, or you have but want to play it again, that you wait for Crossworlds.
Gothic; You can get Crossworlds from GamersGate. It's cheaper than the Steam version, I believe, although it doesn't come with the bonus copy of The Legend.