Debes, deberías (de)

[fglorca]

Which of these below is the correct option?
Are both options correct in any case?

• No debes pagarle / No debes de pagarle
• No deberías pagarle / No deberías de pagarle

Many thanks.

[Tomisimo]

You might read these two threads which have relevant info.

* http://forums.tomisimo.org/showthread.php?t=23100
* http://forums.tomisimo.org/showthread.php?t=3910


I know that I have heard all four options that you give, so they are used, but not interchangeably.