There is a demo of Divinity still available (
Fileplanet, and probably others), though is a little large at 400MB (the beginning/demo and end of Divinity are a little heavy on the hack and slash). Playing Divinity first would give you more of the background and help the story depth of Beyond Divinity, though you do not need to have played the first to get the second.
I'd be tempted to recommend starting with the original, as well, and get Beyond Divinity a year down the road, or whatever. If it was a question of
definitely only one or the other, I'd go for Beyond Divinity. Try the demos, and see if that helps you decide. Both demos are the beginnings of the game and therefor a little linear by design. The Divinity demo has more quests that give a sense of what the full game has in store; quests in the BD demo are limited due to the starting location, but the full game will have more variety and more involved quests.
