So until recently I really didn't have much use for the rapier, but as it turns out my girlfriend has always wanted to learn and so I picked up a couple books to see if I could help her out (if nothing else getting the books in her hands.) Then the other day I started actually READING through one of them (Guy Windsor's Duelist's companion) I suddenly found myself face to face with a very real art beautifully lethal and efficient.
Up till now most of my fencing experience has been with the longsword (Liechtenauer), but looking through this book I began to wonder how I would react to some of the techniques and maneuvers that are presented in the system. In the end I don't know. I think I can see reasonable responses to some of them but I wonder how they would play out, and given a lack of available period rapier fencers I've come here.

So my question is: what sort of move combination/ counters/ etc. would these two swords be likely to use against each other? What would work, what wouldn't that would be tried anyways?

I'm really not interested in "who would win", I'm just curious about the way the weapons would come against each other, and how they would defend.

I train in both Longsword and Rapier. On a theoretical level it seems to me that rapier has grown out of the longsword Sprechfenster (fighting from longpoint) and the 8 windings. The footwork in rapier rather then using more sloping steps (like longsword) uses more gathering steps (fencing advances and retreats). My conclusion is that within the (German) longsword system, you have enough technique to effectively fence rapier with. Its just a mater rearranging how you put it together. This does not mean that all rapier techniques are established in the longsword system, but the basics are there.

In my opinion, context will be extremely important in the comparison.

The longsword is originally a battlefield weapon. While it could be used for duelling out of armour, it had not been specifically designed to do so. The rapier on the other hand has been specifically improved and designed to perform best in these unarmoured dueling situations. Its style of use and design evolved conjointly to fit this context as best as possible. So I would rather think that the rapier is overall better suited to the unarmoured duel situation... How we modern people are able to perform in longsword vs. rapier is another question.

Actually if you look at some of the principles put forward by many blossfechten practitionner (stay covered by the sword during strikes, sacrifice some of the power of cuts in order to wound just enough, take the most direct path of attack), you'll find they are explicitely at the root of rapier systems, and among other why they favoured the thrust.

As an aside, Thibault has a few plates about rapier vs. greatsword. I haven't really looked at them in great detail, but overall the tactic seems to consistently involve binding incoming cuts and get into some sort of long point position where the rapier shines. Interestingly the pure conterthrust is not really favoured, but this is consistent with how Thibault treats the cuts even by other rapiers. And the stepping is not especially linear but this is consistent with the rest of Thibault as well :)

I was presuming both combatants unarmored.
Also as I understand it there was a resurgence in eastern (or central?) europe at a later point to carrying a longsword as a civilian sidearm (specifically as a riding sword) but the examples where all very cut oriented. Afraid I can't remember where exactly (I read it in a discussion on this forum though if anyone else remembers.)

I had figured that winding would come into the equation, but having read on disengages and that sort of thing I wasn't sure how it would go.