Parade Gloss is more expensive, and I think it uses a different solvent to normal polish.
In my experience, if you use parade gloss it is much easier to get a good bull compared to normal polish, so it's great for beginners who haven't developed that magic touch yet. But the resulting bull is nowhere near as hard-wearing as if you use normal Kiwi polish. So once you've got the knack, go for the normal polish every time.
Parade Gloss is a harder polish, so you don't need to be quite so gentle with the final bull. The downside is that it cracks more readily, especially in cold weather, so is most suited to hard toe caps and heel plates, i.e ammo boots.
Normal Kiwi (or just about any other brand for that matter) has a softer shine so, as stated earlier, you need the knack. It's also better at filling cracks and creases.
There are rumours that if Parade Gloss gets wet, you get a blue tinge, though I've never actually come across this in practice.
If you want to cheat, after you've got a reasonable coat of polish on the boots, you can top it up with a liquid polish (I can hear the groans already!) Kiwi Elite (now renamed to "Instant Wax Shine") is a good time saver, but you must let it thoroughly dry (about 6 hours) before giving it a final buff with nylons. In this case, if you get it wet before it thoroughly dries, it WILL turn strange shades.
I find that for bulling, Parade Gloss produces a 'grainy' finish. It is superb for brush polishing though (in conjuction with a rolled up pair of tights rubbed over the boot at the end to add a 'top shine')
Parade gloss is good if your in the training environment and going over your work on a very regular basis, and also if your in the sticky brown stuff and need to do a reasonable job in a short time, however if your doing the job properly I use normal Kiwi and elbow grease! Great results every time!