Since the Tamron does not hold its value quite as well, I would opt for a Tamron if I were going to buy used (and if I could find one). There are actually three 90mm Tamron lenses that have been produced. The first is the Adapt-All version which was a basic lens that used adapters to fit on various manufacturer’s cameras. The Adap-All was not a particularly good lens and I would avoid it. The present Tamron offering is the SP 90mm f/2.8 Di (In the Tamron alphabet soup language – Di signifies that the lens has been optimized for digital). Previous to the introduction of the Di model; Tamron offered the SP 90mm f/2.8 without the Di added to the name.
I have the previous non-Di model and I don’t know how Tamron could make this lens perform any better. My Tamron produces imagery that equals that of my best “L” glass lenses. It is a great lens, not a great lens for the price – but a Great Lens overall. However, I also got this lens used for a Great Price paying less that $125 USD including shipping for a minty lens from an eBay seller. You cannot often find the non-Di models on eBay. I would expect that few photographers would want to sell this jewel of a lens; especially not for a hundred or so dollars. However, if you can locate one selling somewhere between $100 and $200 – SNAP IT UP IMMEDIATELY. It is a great lens! By the way, the beautiful bokeh it produces makes it a great portrait lens.
Oh yes; I checked and there are none being offered on eBay at this time.
So are you saying Non-DI > DI????
How could an older version, be better than newer?
EDIT: Oh, do you mean to go for the non DI because it's cheaper and still a great lens?