Go see Petty is you have to choose. I love Pink Floyd. I admittedly like Gilmore more than Waters. I saw Gilmore in April in Chicago and it was amazing. I went to Waters' The Wall show in 2012 and was completely underwhelmed. His voice is shot.
Petty's voice has held up because his singing voice is just his normal voice singing. Waters always had to push it and use his throat more.
I know it's Petty's last tour. Waters might do another for the money, but that's exactly what this tour is now already. If you want to see someone do a great job of singing someone else's Pink Floyd songs (a lot of the Pink Floyd songs that Waters will sing were originally sung by Gilmore) go see Brit Floyd.