I am replacing my RRA M4 stock handguards with the Surefire M73 handguards.

First off, why use over 'rail panels' over the 'rail protectors'? Or vise-versa?

What companies make the better/best protectors and panels?

THANKS! I did a search and nothing came up.

The handguard can get hot in prolonged shooting, the rail covers shield your hands from it, the foregrip also serves in that function (among other things).
My fav is the KAC rail panels.  Your SureFire handguard has the relief cut on both ends to accomodate KAC rail panels.  I also use strips of ladder rail covers underneath the MRP handguard so when I rest it on a rock or something it won't damage the bottom rails.
For rail handguards that don't have the relief cut for KAC panels, I like the Tango Down panels.

the M73 comes with it's own rail ladder type rail protectors i think.  Mangled rails will do you no good.  Why not spend a few bucks to protect what you've bought with your hard earned money.  Plus you can start color accessorizing like the rest of us lol...

Magpul XTs and ladders are all I use.

They are great products.

another reason--ever try to get mud out of those little slots?

IMHO, rails on personally owned weapons should at least have some type of protection. That being said, consider these points:

Rail protectors are very light weight, are low profile and, because they cover less of the rail, heat can more easily radiate off the rail. They won't really protect your hands against heat if you grab the foreend (unless you are wearing gloves) and rails can still get damaged because they are partially exposed.

Rail panels increase the overall diameter of the foreend (KAC panels less so than Tango Down panels) but they protect the rail from all but the most severe damage. However, they do add a bit more weight than rail protectors and they will cause the foreend to retain heat more than the rail protectors.

Myself, I put rail panels on the bottom and sides and a rail protector on the top rail.

KAC panels are thinner than Tango Down panels but they also cost almost 30%-50% more than Tango Down panels. KAC panels will only lock in to the first or last slot. Tango Down panels will lock down on any slot. KAC panels use metal clips to lock in place and, if these clips are loose, they will rattle. Tango Down panels can also rattle but metal rattling (KAC panel) is louder than plastic rattling (Tango Down).