Actually there's more characters and patches as well:
Difference between SF4 and SSF4: 10 characters were added to the already 25 characters.
And for Arcade four more characters were added.
You're better off getting the Arcade IMO it's $20 and comes with all the characters and you'll likely find more people to play with online as well.
Me, I have SSF4 and have been waiting for ever for MS to dropped it's DLC arcade price down to 800MS points but more than likely just buy the Arcade version at Gamestop. Just checked the site, the consoles versions are still $20 but the PC is now $40 but I would just wait for the price drop on the PC again--it was $20 for awhile and I'm sure it will return in a couple of weeks.