The pair of 3500's should have a bit more displacement potential than the single xxx. But will likely output more distortion as well (xxx is BL optimized and has a flat 'curve', 3500's are not and do not, last I checked). Honestly though, I dont se either difference (displacement or distortion output) being a significant enough change to justify redo'ing the entire system. I suspect any major differences in performance will either be installation based, or perceived changes based on the desire for better performance (buy something new and expect it to do better).
But if you just want to do it for something different, or to just change brands, go for it. Just saying you should not expect any huge performance changes either way based only on the different speakers. Normally I recommend changing the installation/enclosure before running out and buying different speakers, particularly when the differences will be so slight, if/when they are simply growing tired of their surrent setup. IMO you have the potential for more dramatic changes in performance by changing the installtion/enclosure than you do simply by swapping speaker brands/models.
Good luck and keep us posted. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif