I've used KAC, Tango Down and the generic ladder style rail covers.
The KAC are light, thin and durable. If you ever want to use your rail system as a regular handguard, the KAC panels keep the overall profile as small as possible while protecting your hands from hot aluminum. The one thing to be aware is that your rail system must have cuts (on the first and last slots) so the KAC panels will lock in place.
The Tango Down panels are some of the thickest ones out there but they are also very durable and only slightly heavier than the KAC. Regarding weight, it's only a consideration if you're trying to make the lightest AR possible. The TD panels have a rougher surface and feel better (to me) when holding your rail system like a traditional handguard. The TD panels will lock into any rail slot (not just the first and last slots like the KAC) which, in some cases, is nice when you want to securely attach a panel at some other spot. This is really only an issue on mid-lenth or rifle length rail systems. TD rail panels are also available with an integral tape switch pocket which works much better than any adhesive or velcro arrangement.
If you're using a vertical foregrip, the ladder panels may be your best option. They are inexpensive, can easily be cut (with scissors) to fit any rail and allow maximum heat dissapation compared to the other solid panels. A lot of folks use at least 1 ladder panel on the top rail (12 o'clock) since this rail is rarely touched (when holding the rail system like a handguard). It's also more difficult, if not impossible, to be able to slide a solid rail panel on to the top rail and, since we all know heat rises, keeping at least the top rail partially uncovered allows better heat dissapation than a solid panel.
If you need to cut any of the rail panels to fit, either use a small hobby saw and mitre box, a Dremel w/a cutting wheel or some type of mounted electric saw. Don't both buying anything but full length (7") rail panels because as you will find out (if you have not already), the shorter panels cost just as much as the full lenth ones.