There's all kinds of things to factor into a decision like this-
1. Purpose- what are you trying to do, and why are you doing it. If you're just trying new and different parts- derailleur changing is easy and it fun and there's zillions of choices and combinations- totally recommend playing around and learning. And "because I want to" is a totally legitimate reason. But- if you're looking to fix a problem or upgrade what you have going on- it's important to look into why you want what you want- before you drop the money into it. Which leads into....
2. Performance- I don't think there's a difference between an ARX and a V-GT Luxe. I've used V-GT, V-GT Luxe and an AR (not X). Just like all Suntour derailleurs- they work well and smooth and everything.
3. Weight- I'd guess about a horse a piece... again pretty even. These were touring derailleurs... they were meant to be as strong as they needed to be and consequently, as heavy as they needed to be. A Cyclone weighs a lot less- but it's not as bulletproof.
4. Aesthetic- this is where the difference is- IMO there's an aesthetic difference between 70s and 80s bikes. Again- IMO- a V-GT Luxe is an older looking piece but performance is right up there with the best of 'em- but the design is literally 10 years older than your bike and sort of anachronistic. If'n I were going to upgrade an 84-ish bike like yours I would be looking at a Suntour XC (not XC Pro, or XC Sport- just plain ol' XC).
I seriously don't mean to talk you out of anything- ESPECIALLY if you're doing this for science. There's really no wrong way to go about it, unless you break stuff. What better way to find out that a Superbe Pro won't work with a 28 tooth cog than to find out that you really can't get a Superbe Pro to not work on a 28T cog?
. (and some people have gotten Superbe Pros to work on 28s, but not me).
Again- this is just my stupid opinion- I was trying out all kinds of long cage derailleurs with my 1985 Trek 720- Pretty much everything I could get my grubby lil paws on. There were 2 derailleurs that didn't work well for me, both Suntour- a Le Tech and a 2nd generation Mountech. Oddly, they were both triple pivot derailleurs sort of like the Duopar. The VG-T Luxe worked great- but it looked old and kind of big and blocky. Conversely the Cyclone I used was the old style- and the aesthetic didn't bother me a bit- it worked fine- but it felt so dainty. I'm not a strong rider- but that derailleur felt wrong on that bike. So then- in sheer defiance of my aversion to big and blocky and heavy- my favorite friction derailleur on that bike was a triple pulley XC. Go figure.
Suntour XC by
Dave The Golden Boy, on Flickr
2nd Generation Mountech by
Dave The Golden Boy, on Flickr
I don't think I have any pix of the bike with some of the other derailleurs on it- if I did, they were probably swallowed by Pee Bucket.
Suntour V-GT Luxe Rear Derailleur by
Dave The Golden Boy, on Flickr
Suntour Derailleurs by
Dave The Golden Boy, on Flickr