In article , eudani@jorsm.com says...

>Is there a difference in clarity and contrast between the two? If I have
>the 3.5 version, and I saw a used 2.8 in good condition, is it worth the
>trade?

My information/memory about this could be faulty, so keep that in mind...
I recall that the Minolta 135mm f3.5 was unusual in that it had 6
elements, rather than the more common 4 for a lens of its type - and my friend's sample checks out quite sharp. The gain of 2/3 stop or so in
speed may not (or, may) be worth the trade (speed is always useful with
longer lenses), but it is likely that you already have an excellent lens.
Hope This Helps