Depends on the band. Sure, I love Propagandhi's first album. It was one of the first punk albums I ever listened to (along with NOFX's Ribbed). But every album they've put out since has been better. Every single one. So if we were talking about Propagandhi my answer would be "new one, any day."
What about The Weakerthans? They're my favourite band. Fallow had its moments and Left and Leaving was stellar. And sure, one might say they followed Propagandhi's theme and kept putting out better records with each successive release, but I'll always go back to Reconstruction Site. Always.
And that other band I mentioned? NOFX? I haven't even listened to all of Coaster, and Wolves in Wolves' Clothing was terrible. The War On Errorism was okay, I suppose, but Pump Up the Valuum was medio-core. So if we're talking NOFX my answer is "old stuff, but..." And, you see, that "but" is there because their old stuff -- Liberal Animation and even S&M Airlines -- f**king blows. Ribbed through So Long and Thanks for All the Shoes is about the only portion of their catalogue I'll listen to (with the notable inclusion of The Decline).
Or what about The Fullblast? Okay, so they're not making new music any more, but I guess the question would be first album or second? But I've never separated the two, having bought them on the same day and listened to them back-to-back for as long as I can remember. Their entire catalogue is what it is, neither old nor new to me. And I love it all.
And don't even get me started on The Beatles. One might argue that it's all "old stuff" but I like a shitload of Beatles tracks from every era.
So I guess my answer is that this question was about as insightful as asking me to rattle off a bunch of musical genres. I just like music. The end.