I have two very good friends who are both Guros under Tuhon Jerson "Nene" Tortal. They hosted Tuhon Nene for a seminar back in June 2002 - which I attended.
Just for the record, I am a student of Pekiti Tirsia under Tuhon Bill McGrath. So I can only speak of the difference between the two arts from the perspective of Pekiti Tirsia as taught by Tuhon Bill McGrath. With that being said, the two arts are really nothing alike. The basic strikes, the footwork, and the different strategies for close, middle, and long range are very different. The only striking similarity is that they share a drill but even the particulars of that are different.
They both move very differently as well - this is pretty evident on the Contradas video tape from Lionheart. For those that have seen the tape the drill that is shown for single stick/sword exists in all three arts - Pekiti as done by Tuhon Bill McGrath, Pekiti that is currently taught by Tuhon Leo Gaje, and Dekiti as taught by Tuhon Nene Tortal. Of course the emphasis, footwork, and place it has in the overall art is (I'm pretty sure...) very different.
Also, at the seminar, Tuhon Nene defined his art's name as follows: Dekiti - close range; Tirsia - options in fighting; Siradas - closing the door (to stop an attacker or to not allow the attack to penetrate...). I hope this helps.
-wes tasker