The story behind the tapered Fox needle is that some time after Duke Fox died, John Lowry started taking the Fox van and Betty Fox to various contests. She noticed that almost everyone using Fox 35s in contests were not using Fox needlevalves. Shortly after (1993-ish) Fox started handing out the new tapered needle for testing, along with instructions that Fox users could chuck their old flat sided needles in a drill and attack them with a file until fully tapered. Aside from the extra grinding and at some point a slightly finer knurling, the full taper needle is the same as the flat sided needle. About a year after the test needles, it seems like all the needles coming from Fox were the tapered version. I want to say the last 40th Anniversary Fox I bought (the 50th's were already out on the market, about 1999-ish) came with the fully tapered needle, along with the finer knurl pattern and barbs on the fuel line end of the spraybar.
So how well does it work? Better than the flat sided needle. Trouble with the flat side needle is that if you can't find the happy needle setting within about half a turn, you have to go another half turn in a completely different setting, before you get back to kinda where you were before. Basically you could turn the needle in, and it'd go from a rich setting, to lean, then rich, then lean, etc. In theory with the dual hole spraybar it's not quite a 1/2 turn, but you get the picture. By making the taper round, instead of a flat spot, suddenly the Fox needle is linear! Go figure.
So what about the spade tipped combat needle? Hard to say exactly, but it probably went from the spade tip to a fully/fine tapered needle because it was cheaper to manufacture, and not goofy. There's also the fact that the early Combat Specials shake like hell, while the later ones did not. The Combat Specials I run on my planes all get the later full taper needle, mostly because I've got a ton of them stashed away and wouldn't feel terribly bad if I accidentally damaged one in a crash. They work fine on the Series III, 36X etc. My Series I and II kind of run like poop, but I really doubt that has anything to do with the needle.
There are other problems with the Fox needlevalve. In no particular order...
1. The threads are OK but the tolerances are Fox spec. You can get a needle that leaks air into the spraybar and that can be a problem. The easy fix is to cut a piece of fuel line about 5/16" long and slip it over the needle before threading it into the spraybar, so it compresses between the spraybar and the lower knob on the needle. This will also dampen some of the vibration.
2. The next issue is the Fox needle is fragile. They'll break on nose overs, or catching a loose fingertip on a leather starting glove, etc. When there's a chance it'll get in the way, and it could stand to be shorter the easy fix is to clip the top knob off the needle, then grab the stub just above the lower knob with pliers, and bend the end sticking past over about 90*. The steel is soft enough to make the bend with pliers and I've never had one fracture in the process. I've never managed to break one shortened as described, and it has the side benefit of making it easy to see where the needle is pointing and easier to adjust.
3. The spraybars without barbs don't like to hold modern silicone fuel line. There are a couple fixes, you can try to find smaller "medium" size fuel line. Some manufacturers make a smaller medium size that tends to bite the spraybar better. Another fix to try is cutting another piece of fuel tubing about 5/16" long and stretch it open with a hemostat and slip it over the end of your main fuel line, which makes the tubing bite a little tighter. A person can also solder almost anything on the end of the spraybar to act as a barb, just keep in mind it'll have to be smaller than the threads or removed to change the spraybar in the future.
4. Sometimes when you put the modern full taper Fox needle in the older brass spraybars, the needle won't screw in far enough to close the fuel jet in the spraybar. Whether this is because of tolerances, or just one or two bad needles, or bad spraybars, I don't know. All I know is that sometimes you can run the needle in as far as it goes and the engine still runs rich. The old flat needle works fine with those spraybars, and the new tapered needle works fine in other spraybars, including some of the brass ones. I'm not saying you'll ever run into one that has this problem, but it's something to watch for. It's easy to check, just slip a piece of tubing on the spraybar and blow in it while you tighten the needle. It should get harder to blow into then stop flowing completely.
The ST NVAs are great. Set the collet snug, with the needle still easily adjustable. When you get it close, tighten the collet until you can turn the needle without bending it or hurting your fingers, and you'll probably never have to touch it again unless you change the setup or the conditions you're flying in.
I run mostly the late tapered Fox needle, in both the old brass and later steel spraybars, modified as above, and have very very very few issues. I've got several setup with the ST G51 NVA and they run fine too, when I have to make an adjustment it's easier to fine tune the ST needle, but that's usually just the first few flights on a new plane, or after removing the needle for maintenance, etc. In some cases, where it really doesn't matter (Bi-Slob) I'm still using the flat sided needle. Since it doesn't matter, and I rarely ever need to make an adjustment to a Fox 35's needle, they work just fine. Given something other than just a sport plane, I'd probably choose the Randy Smith NVA for the Fox 35. The only reason I don't use them is that most of the stuff I use Fox 35s on aren't worthy of such a nice piece of equipment.
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