Lithium-ion batteries pack a lot of power into a small area, which increases their potential risk compared to other types of rechargeable batteries, so it's smart to learn the simple ways to safely handle and use them.
Here's a great place to start:
Battery University:
http://batteryuniversity.com/
The main points are to NOT overcharge the battery and don't let it discharge to much either.
Protected cells have a special circuit added to help prevent either of those two situations. Many lights also have a special circuit to cut off the light if it goes below a certain voltage.
By the way, lithium-ion cells LIKE being charged frequently, so it's ok to "top off" your battery as often as you like, which means you can always leave the house with a fully charged battery!
Finally, don't leave any lithium-ion cell charging unattended, and always place the charger on a non combustible surface "just in case". The chances of anything happening are quite low, but an ounce of prevention..............
Don't let all this scare you away from using lithium-ion rechargeable cells, as they help provide us with super small lights that can really pack a punch, like the Eiger X..... and with only minor precautions they can be used quite safely.
As to the lug vs keyring option. The lug body will be shorter and the loop is permanently attached (so no tail standing), while the keyring body will be longer and the attachment point actually screws out, which allows both tail standing of the light or adding an optional screw-in mechanical momentary switch.
Some models also offer a "Pocket" body which has no split ring attachment point. This is usually the shortest of the 3 bodies.
For your best info on all Peak lights, go to the RMSK website during the day. Bob is usually available for a LIVE CHAT, and he will happy to answer all your PEAK questions.