The navigator from woodcraft/baron from whoever is the smallest kit that uses metal to nylon threads between the cap and the body. All the variations of the flat top/traditional/american and the round top/euro kits use a metal to metal thread (at least every one I've ever found). Some people don't think this is a big deal, but the buying public seems to vote (with their money!) that it is. I'm not sure how the cap attaches on the stretch that Aaron sells.
There is also a significant difference in the FP sections that come with the kits. Some use a plated or enameled metal sleeve that the plastic insert threads into. Others use a one piece plastic section that the nib and feed just slip into. These in my experience do not hold the nib in contact with the feed as well and may cause ink feed issues. I have also personally had issues with these plastic sections breaking at the thread where they screw into the coupler. Needless to say, anytime I order a new fp kit now, I CALL AND CHECK that the fp section is metal, and not all plastic. It seems that berea makes and sells different components to different vendors in the same pen kit, so don't assume that just because the euro FP you saw somewhere at some point had a particular section, that the ones you order from supplier x will have the same one. The kit nibs are all pretty much the same, made in China, marked "Germany". I have had some that no matter how much I messed with them never wrote worth a darn. On the other hand my favorite FP section has a chinese nib on it, and it writes better FOR ME than any other one I have made or used. It is pretty generally accepted that high dollar fountain pen buyers look down on them, but those people generally aren't looking for the slimmest fp they can get either.
However you end up going with it, be sure to take the section apart, check the fit, look for any burrs or other stuff that isn't supposed to be in there, reassemble, ink it up with some decent ink, and try it out. Write a couple of pages worth with it at least, and see how it works. If you don't like it, take it apart, clean it up, read Lou's tutorial and put it back together and try it again. repeat as necessary. When you are finally finished, flush the nib out with windex till it flushes clear, then clean water, then blow out with compressed air if you have it. Reassemble and deliver to new owner with confidence that it will write as well as it's supposed to. Nothing more embarrassing than having a customer bring a fp back and tell you that it leaks ink everywhere because you didn't take it apart, check it out, and test it first. DAMHIKT Some people have said "a inked pen is a used pen", but better to call it "quality tested" than have it come back (or worse, not come back but the new owner tell all his friends) because of a performance issue. YMMV of course.