I own both.
First of all, the 501s are open and the 271s are closed which effects sound. The 271s have more bass as far as my ears go. But they lack the mids and the highs of the 501s, and cannot touch the 501 soundstage.
My preference is always to go for the 501s, but using an amp. They need one, in my opinion. I have the SM V.3 and I use the bass boost which really brings out the 501 tight (and somewhat shy) bass.
The 501s need a lot of burn in, upward of 200 hours plus, at least for mine.
I use the 271 when I need some isolation. They can get hot around the ears, but I am always pleased with the sound....after a while I then always hanker for the 501 soundstage, mids and highs!
I'm happy to own both.