A rich and varied tale and time period it may be, but the late 1400s and the rise to power of the Borgia family does not seem like material that would make for two (quite different) series.
The Borgias on Showtime went for a bigger name at the head – Jeremy Irons – and aimed for more salacious and sexual (although Borgia is not far behind). Not unlike the other period pieces Showtime has pushed. Like history won’t be interesting unless it’s presented with boobs.
Borgia is something different – it actually takes a bit more time to explain the context in which the story fits into the world. No huge names here – and it is hard to buy John Doman as Rodrigo Borgia. He’s really such a … cop or corrupt cop or bad guy, you know. He was central to The Wire, and in general is just so American that it is not easy to see past. He shows up everywhere and in a lot of different roles, but as a cardinal/pope in this particular time period? No. Jeremy Irons pulls this off, and while Doman’s a talented guy, he is a man out of place here. The rest of the acting is terrible – an international cast that speaks questionable English even though it’s an English-language production… heavens, please.
I cannot say, even though I sat through both series, that I would recommend either.