this will cover it.. it's a link so click on it
I am sure, going by your signature line, that you are familiar with the datecodes......
So simple .... anything before "AE" (2004) that is marked P99 is the now called P99/AS which is a DA/SA.... the action is unique as when you rack the slide to chamber a round the trigger does not move in it's SA position (like many other Walther's) but stays in the DA forward position.......you can either then decock the P99 or have the long triggerpull with no resistance till you get to the SA pull... hope that makes sense.......
then there is the P990 which you already mentioned... also changed destination, to P99/DAO in 2004... however you won't see many around of those post 2004 DAO...
and then there is the P99/QA. sometimes also referred as a DAO (on auctionsites and even Waltheramerica site)... It is kinda like a Glock with a precocked action, however it does have a decocker. Which is there so you can take the P99/QA appart without having to pull the trigger.... if the P99/QA is decocked it is rendered "useless" untill you rack the slide about 1/8 of a inch back to re precock the striker
hope that helps....
I am not going to say one is better than the other... since that is a personal choice.. however I feel if you are looking for a classic... you will get a P99 or P99/AS..... after all Walther stands for DA/SA autos IMHO