I find I like more and more OT figures... The diversity is more appealing in those films, whereas the PT figures are only starting in the past year to get great...
I look to the PT for more great main characters and I feel they really can have the upper hand there, as characters like Windu, the various stages of Obi, Qui-Gon, etc. are all lacking great figures... I don't look to the PT much for weird background aliens and whatnot though... I'd take Cantina aliens over the Senate or Pod Race any day...
PT army builders are limited to Clones and droids for the most part... I have more interest then in OT army builders getting their just desserts... The Rebels are really getting the short end of the stick for quality troopers, and let's face it that you can get Clones in abundance but Stormtroopers of any great quality are a tough find...
The OT needs its main characters too, but they seem to have more... VOTC's helped the OT in that regard. The PT seems to just need a LOT of main character resculpts while the OT just needs a hair fewer... Still, the other reasons listed have me leaning towards the OT being my favorite...
ROTS is where I really want the most PT work, but the OT I think needs new figures from all 3 films equally... That alone is reason enough to lean towards the OT in my book.