All things being equal (which of course they never are), original, or even close to original, pressings are going to sound better than vinyl re-issues almost all of the time. But it will come down to what your tolerances are for surface noise, ticks and pops etc. I will tolerate a bit of that (but NOT groove damage) to find a nice original pressing, generally buying from dealers who will offer a return policy if not satisfied. In terms of buying originals off E-Bay, etc. I have not really ventured into that area and I would only consider it if the seller had a very high # of positive feedbacks relating to record sales or came personally recommended from another record buyer.
Jazz originals in good shape are generally hard to come by and not cheap; I've been quite happy with most of the OJC re-issues I've purchased and very happy with the few Speakers Corner re-issues I've bought. With 70's-80's rock, very often cheap original or close to original pressings in good condition are available at reasonable prices that put the re-issues to shame. I've recently picked up copies of Dire Straits debut album and Pink Floyd's DSOTM that sound much better than a Japanese pressing of the Dire Straits and the 30th anniversary re-issue of DSOTM. The originals were purchased for 50 cents or a buck at the thrift vs. $25 U.S. for the anniversary DSOTM.
Once you get into the 50's and 60's, though, it's (at least in my experience) much more difficult to find quality originals or close to originals. I just bought a re-issue copy of John Mayall and Blues Breakers with Eric Clapton only to discover it's from a digital master (no markings anywhere on the cover or label indicating that, but it sure sounds like it to me), so I'm contemplating biting the bullet and ordering the Speakers Corner on that 1966 album because I really like the performance.
