Flexibility and CustomizationCustomization of your overall game experience is where Myth really shines. Players decide game length, difficulty per tile, which quests to use, which of three game modes to play, whether or not they're ever going to fight their least favorite monster again, etc. How many and which monsters are in the room? Myth provides you with a range, not a precise answer, and you tweak that sliding scale depending on how much challenge you want, and what seems to fit the narrative. You are your own GM, and you get to adjust everything on the fly. If you’ve got a little creativity, Myth gives you a ton of tools and options to explore. Existing quests have a lot of variety, and it's pretty easy to add your own ideas to the deck if you've got the cards sleeved. (Speaking of which, I've made about 20 new quest cards that I'll be posting to this site sometime soon.) All this comes with a couple caveats: some folks find Myth too unstructured and nebulous, or feel that making the decisions Myth asks them to is tantamount to cheating. Myth's philosophy is pretty unique.
Galaxy Defenders really relies on detailed scenarios, and it’s hard to eyeball how they will play out before you try any specific scenario. It’s also much harder to make your own scenarios for GD than for Myth, and all that hidden information makes customizing an existing mission a little tricky unless you're really familiar with the particulars. Scenarios feed into each other in a specific (branching) order, with expectations of equipment and level-ups happening at a precisely controlled rate. It’s a very scripted experience. In classic RPG terms, I’d be willing to bet that Myth appeals more to people who like to GM, where Galaxy Defenders appeals more to those who prefer to be players in the RPG party. If you do want to add your own content to GD, the unique components (equipment and abilities are printed on thick hexagonal and trapezoidal tiles, not standard cards) makes it a little harder.
Shadows of Brimstone falls somewhere between them. You can easily choose to alter or ignore certain decks and Other Worlds to tailor your game, so it’s more immediately flexible than GD. You can just wander into the mines and face whatever you draw, but at the demo I played the designer heavily implied there were going to be additional more complicated missions in the final game. Accessories to the game include blank cards so you can insert your own ideas into the pot. However, there’s a lot of hidden information and decisions made by random card or token draw. By default, SoB just doesn't give you nearly the level of control that Myth’s free-questing and realm tile setup allows. All your customization has to happen between games, where as in Myth it can be done on the fly. In SoB, the decks function as your GM, in Myth you are your own GM. Which means that if Myth seemed just a little too unstructured for you, SoB’s tighter dungeon-generation system will probably be very appealing.
And The Winner Is: Either Myth or SoB, depending on your personal tastes. Both are more readily adaptable than GD, but that comes at a price. They rely on random card draws instead of GD's carefully scripted adventures. You get flexibility, but at the cost of sometimes have to deal with incompatible card draws. Myth has a built-in solution to that problem (the players are specifically empowered to disregard the cards if they want) but that could itself lead to analysis paralysis or just rub some player's sensibilities wrong.