itch.io

Hiii Vincent, thank you for asking! You ask a very relevant question indeed and I hope you don't mind if I go a bit in-depth about that: I will also add this answer as a FAQ on both platforms.
As a short answer: the 20% discount (30% for Lv.4 Geomancers) will be for future stand-alone or bundle asset packs, separate from the Adventure Series "starter pack".

An in-depth analysis:

• Adventure Begins and its two major expansions will always be included as base perks for patreon tiers 3 (Architect) and 4 (Geomancer) AND in the 14,99$ itch.io price tier.
  1. The first expansion pack is called "Adventurers & Adversaries", features overworld characters and enemies will be available later this very month: https://zaebucca.itch.io/adventure-begins/devlog/681149/adventure-begins-adventu...
  2. The second expansion pack is called "Adventure Below" and its two instalments (tilesets and character sprites, much alike Adventure Begins) will release mid-March and late-March.
  3. A third, final expansion in two instalments (tilesets and sprites as usual) releases in April, completing the Adventure Series.

• After the Adventure Series reaches its completion in April, further assets will release monthly and simultaneously on itch and Patreon. These assets will either be further expansions / concepts for the Adventure Series and brand new series or styles such as side-scrolling, isometric, sprite packs etc etc.
  These packs will be included in patron's subscription for patrons as one-time, monthly codes at no added cost. Patrons, among other things, also get to partecipate in the process and give early feedback / ideas fore the new material through polls and workshops.

• After the monthly code expires, these packs will still be available through itch.io to purchase.This is where the 20% discount (30% for Geomancers) comes in handy: bundles, legacy packs and any purchase that are not featured in Patreon's monthly release for new subscribers will be discounted anyways for patrons.

• Please note that in the future, there will be new itch.io bundles of course, as well as promotional sales and discounts and the possibility of creating/adding new material to patreon starter packs other than Adventure Begins. Of course, the new material and timed perks will always be considered benefits to patrons and early itch.io supporters!

So, to sum it up:
Itch.io is tailored around developers or fans who would benefit ready-for-use, specific assets and resources once they are released/updated, whereas Patreon is meant as a long period comunity project that offers added perks and interaction (like personal art archives, HD collections, wallpapers, tutorials, discord, workshops and so on), with the added benefit of featuring new monthly, timed releases from itch.io.

Please let me know if there are any doubts!