depends on what your intended use is, in my opinion.
i’ve owned a 64 from 2008, a 256 from 2012, and a 128 from 2016—here are a few points that stuck out.
the aluminum 128s are undeniably excellent—leds are beautiful, build-quality and durability are outstanding, versatility is nigh-maximal, and perhaps above all else, the buttons are incredibly playable. i use mine primarily in tandem with norns, as the keypresses feel just as natural when improvising on earthsea as they do chopping breakbeats with mlr.
the older models, however, are incomparable in their specialized functionality—if you’re really big into mlr like me, for example, you’ll likely be sorry to have anything other than a 256 with tilt controls and maybe even a 64 as an aux. while the buttons on my 256 in particular don’t feel quite as musical as the ones on the 128 somehow, simply having more of them makes it the easy choice for a monster mlrv set in performance—to me, it’ll always be the biggest and the baddest.
at this moment, i use the 128 most. when i was playing professionally, it was 256 all day long.