The thing about Penn's performance is that, ultimately, it's not but a serious exercise in acting. Seriously, yes it's great, but what did it do for you? It's another bio role, it's another part where the best thing the actor does is transform itself.
Rourke's role was more about living it, and you sense it, he doesn't have easy things like quirks to hang on to, not he's acting was all about transmitting emotion, living it, making it look real.
I see rourke in wrestler, and it makes an impact in me, i can feel his pain at times. I see Penn in milk, and i'm like "Well, he does act really goddamn gay." And that's all.
There will be another acting performance like Penn in milk in the next years, time and time again.