I just started buying a bunch of replacement windows to do my house needing 21 total. I was planning to do all of the with the pella so started with 3 pella thermastar 20 series windows and installed them. then I had to buy a couple reliabilt 3500's due to a certain style that i needed for the next 2 windows (cottage style sashes). The 3500 are much better in my opinion. I just bought 10 more and switched to the reliabilt. They are sturdier, lock tighter, have less vinyl frays from manufacturing/quality control, and have a much better installation conceling mechanism (pella uses little foam sticky pads while reliabilt uses continuous inserts all along the sides to cover the screws and adjusters). Also, the sashes on the reliabilt are smoother and the locks are sturdier. The pellas use the old strings for balance while the reliabilt uses a more concealed tape mechanism. Not sure what better, but the reliabilt seems nicer. Even the screens are better in that the pellas have little black knobs that are clearly visible. I can't speak for performance yet, but i recommend the reliabilt's based on side by side initial comparison and operation.
The Reliabilt 3500 double hung replacements I just bought from Lowes are AAMA certified --from what I've read, this is a pretty good certification that not all window manufacturers are willing to apply for.