Choosing between Reissue and Original pressings

First pressing? By that Pani I assume you mean the first stamper.

If so, the desirability of the first stamper escapes me, as with the RCA S-1s. It would seem the critical issue would be how many records a given stamper pressed. So would the fifth pressing from S-10 be worse than the 1500th from S-1?

But I don't know how many records S-1 stamped out, nor do I have any idea how to even guess which pressing a given record might be within its respective stamper's life.

So bottom line, it becomes a matter of auditioning any two pressings of the same record. And yes, I have heard significant differences.

I don't know if there is a general rule of thumb on which pressing or country of origin is best. Others with more or different knowledge are free to contradict me, but here's what I know:
on certain records, I suspect the first pressing is more desirable for collectability than sonics;
there's all kinds of lore (some of which may be true) about the matrix numbers, pressing plants and mastering engineer inscriptions- I certainly couldn't generalize about that, or point to particular examples where I have compared one old record to another and concluded that a later pressing from a different plant sounded better than a first pressing; I know there were preferences among RCA collectors, but I haven't done enough comparisons to make a valid judgment. I do remember comparing a white dog to a shaded dog of one old RCA classical record and the white dog sounded better- this was quite a while ago.
as to country of origin, my guess is to look where the record originated from- Mercurys are probably better from the US; UK stuff, like Island, probably UK. I think out of all the Pink Floyd Dark Sides, it is the UK original that gets the votes for sonics.
As to Japanese reissues, I think the key there was that a lot of records were made of crap recycled vinyl in the late 70's-80's, and the japanese were touting virgin vinyl. But, given what they were working from in the way of masters, I wouldn't necessarily say a japanese reissue is hands-down better.
Does your last question mean, can you hear the difference? I gave you a couple of examples in my earlier post. And the originals were pretty standard issue, generic pressings from major labels of big selling popular records.
As time and my system has improved, I have found most of the MFSL records (old ones that I bought new back in the day) to sound very average, nothing special. I think most were mastered at that time by Stan Ricker. This was true for the UHQR as well as the 'regular' 1/2 speed masters, of which I have quite a number.
On Rumors, I really never grooved on later FWMac, i do dig the early Peter Green stuff, but the copies I have of Rumors are original issues as far as I know (I will double check for you), and they sound pretty good.
I love Muddy Waters, but I never got why the Folk Singer record was so highly regarded musically. I prefer his smokey electric blues thing, but that's a music content issue, not a pressing issue.

Pani, try this website for some insight and strong opinions about the subject. Tom Port is considered a bit nutty by some here but I think he knows more about quality pressings and stampers than anybody. I also buy a lot of records from him.

http://www.dccblowout.com/

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GMC, the problem with better records is that there are no guidelines other than his ear in picking out the good sounding copies. he trashes most of the well-known reissues which may be right, but after that, he provides zero info on particular release versions, stamper numbers or other indicia of what particular copies sound good. Aside from paying 499 or more for a garden variety mass produced major label issue. Hell, for 500 bucks I can buy quite a few copies and do the same thing. Time-saving, perhaps. And funny to read his comments. But not sure there's real, useable positive info there on what to buy, more negative on what not to buy (which, as I recall, is pretty close to everything except those selected by him).

Whart, I agree that there are no guidlines, anywhere. That is my point. I don't think there are any clear rules or guarantees other than our ears. I pay top dollar to get the copies of the records that are important to me. While I might try to do it myself, time and access to quality copies would make it very difficult. Each copy is going to sound different.