The differences have been explained above. As many will tell you, you'll simply have to buy a tin of each, clean barrel in between switching pellets, and shoot groups BR @ 20-30 yards on a winless day. That will tell you which pellet your barrel prefers, and if you have a chronograph, try each pellet at different speeds throughout the test.
You should also consider taking a pellet that you know is slightly tight/oversized. Shoot it at the lowest power you can into a 55 gallon drum of water or simply push it through if you can. You may have to remove barrel. You want to push from breech to muzzle as chokes vary. LW uses a 0.004" so you can add that for reference.
When you load, you want the pellet seated as in ready to shoot. Gun NOT cocked, remove spring if you can for safety, and safety on of course. You'll be looking down barrel with a very bright flashlight at loading port. Checking to see if you can see any light around the edges of pellet. Do this with no pellet also to check probe diameter seal.
Either way, light or no light, the skirt will be blown out and ride the L&G's. Ideally the head will engage just enough so it's truly concentric in bore. This is where FX Smooth Bores pull ahead. Less deep marks on pellet equate to better accuracy.
Buy oversized H&N FTS/FTT & consider buying a set of these.
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http://www.airriflet
I just saw a Beeman .177 sizer with 3 size dies. Guy was asking $168.00. I saw an exact on an auction site with one die, $161.00. For around 19 Euros each, you can get the size you need for all calibers for less.
