There a lot of minor differences in the various rules. To me, and this may not be the same for others, the biggest differences is in the bookkeeping and number of reaction tests - in other words the amount of abstraction applied to have a satisfying game experience.
WHAA is currently the oldest set. Characters have more stats to track, there are more weapon stats, the magic system is more detailed in spells and spell descriptions, but not seems a little clunky compared to the other rules. It has rules for mass combat that are similar to the old Warhammer & Battlesystem rules. The big selling point of this rule set over the others is probably the dungeon creation system. While it has character and army lists for a large part of Talomir, it focuses on the Border Kingdoms.
RRtK was the next fantasy set, which is strictly a massed battle set of rules. To me, it is what DBM, DBA, HoTT & FoG should have been. Not only is it a great set for fantasy, but it is excellent for historical games. For the RPG player, this set has the most complete map of Talomir with descriptions of each country's alignment, type of civilization, and composition of armed forces.
Next came RSBM, which is gladiatorial combat. This set introduced "signatures" or attributes to the game. Being a game of individual combat, it has the most detailed person to person combat rules.
WHL has smoother mechanics and reaction tests than WHAA. There are few character stats to track, but the stats that were dropped were replaced by a more robust set of attributes to add more dimension to characters. This set uses the "in sight" mechanism. Whereas WHAA uses dice rolls to construct a dungeon, WHL uses a deck of playing cards. The game focuses on Mireholme, Altengard, Capalan & Tereken for settings.
After WHL came the updated LoA, which is a campaign for 2D6 Sword & Sorcery which is included. This set introduced the Increasing and Decreasing d6 in place of experience points or gold pieces to simplify bookkeeping. The part of Talomir focused on by this set is Barylistan, Demeskeen, Kurinthia, Riata, Sortium and Zamba.
WHW is another massed battle rule set for small forces - Warbands, raiding parties, etc. It has siege rules. It is based in part on the old Montjoie rules. It has campaigns that focus on Mirholme, Altengard and Capalan.
Next was 2HDC, which you know about.
Talomir Tales is the latest set of fantasy rules. It departs from WHAA & WHL in that you do not need a complete tabletop of terrain to play on. The "in sight" mechanism & many of the reaction tables have been replaced by the action table mechanism. It also uses the increasing and decreasing d6 mechanism for character advancement. The game is bundled with a campaign or set of adventures. You can by one set bundled with a campaign and thereafter buy only the campaign modules you are interested in. This way, more of Talomir can be explored and for gamers with a busy life, a lot of the set up work is done for them. Of course I'm going to shamelessly say that Piathoe's Peaks is the best one released so far. The author is top notch and knows her stuff! Ok, if you haven't guessed it, I wrote it.
I hope this helps. Comparing WHAA, WHL & TT is a lot like comparing D&D, AD&D, Pathfinder, & D&D5e, a lot of similarities but enough differences that gamers will develop their own preferences.
Though it took me a while to warm up to the battle board, I can safely say that I've enjoyed every THW game I've ever played. Even though I was stranded on a deserted island by a back stabbing ship's captain, even though I was kidnapped by great apes just as I laid my hands on the treasures of Opar, became a successful boxer going done the rabbit hole on drugs, met a horrible death in Lemuria - I've enjoyed every game.
P.S. The THW Vault has Warring Fleets - a great naval fantasy game with even more bits of Talomir in it.