Well, I've unfortunately never played the first game (Sengoku Ace), but I do have the Saturn release of Sengoku Blade and I highly recommend it. Psikyo were really in their element developing this side-scroller. So for the second game alone, I'd say it's worth picking up (if you don't already own the Saturn rev).
And come to think of it... is it just me, or do only the Sega releases of their games come with the bonus characters (Gunbird 2 and Sengoku Blade, right off the top of my head)?