To me, Powerslave is stronger. But it's not as easy as it seems.
When I was in high school, Piece of Mind was easily my favourite Maiden album (along with Brave New World... and AMOLAD, which came out later, so it doesn't count).
I loved To Tame a Land (a great epic, and I liked the idea of a song based on Dune, which I loved then), I loved Revelations (still do, one of my favourite "early" Maiden tracks up until now), I loved Where Eagles Dare, I loved Die with Your Boots On...
However, the years that passed have not been kind to the album - I can see now most of the songs are a tad filler-ish. Quest for Fire, Sun and Steel, The Trooper (which I've never liked that much)... Even Icarus (a song that doesn't sound that much like Maiden to me, to be honest)...
Powerslave is definitely stronger. I mean, Aces High, Powerslave and Rime are all ten out of ten material, as far as I'm concerned. I even like the rest - it's all fun. I like the solos, I like the "dry" sound and I like the way the songs stick together and give the album a bit of a cohesiveness (POM had that, but only in the ways of sound... the songs on Powerslave fit together much more). The Duellists is a forgotten highlight - it's a shame it's never been played live.
So, considering the original question - Powerslave is better. I like Piece of Mind, just as much as I like every other Maiden album, but Powerslave is probably one of the strongest albums in the 80's by Maiden. Okay, so maybe Somewhere in Time is better, but it's a close shave anyway...