What's the difference between the beta access of Landmark's founder packs?

The page you linked actually details each pack, at the bottom. It is entirely possible that without the correct plugins, the initial pack prices will appear, without the ability to directly purchase the pack and the list of included benefits.

Please keep in mind that two of the packs are upgrades. If you buy a more advanced pack, you will receive all the benefits from the previous packs, as well. While still available, you have the option to upgrade an existing pack here.

In regards to the closed beta rollover text, both the Explorer and Trailblazer packs advertise "Be one of the first to play Landmark as we enter the closed beta period", while the settler pack advertises", while the Settler pack advertises "Be among the first people who get access to the Landmark Closed Beta with unrestricted Closed Beta access". The observations that these two wordings say exactly the same thing are backed up by the fact that The Settler Pack is the only pack to use alternate wording, but is included in both The Trailblazer Pack and The Explorer Pack.

The Settlers Pack

• Unrestricted Closed Beta Access: Be among the first people who get access to the landmark Closed Beta with unrestricted Closed Beta access.
• Settler Flag Item: An exclusive flag item you can place that proudly shows off your status as a Settler to everyone! We're even going to include a recipe that will allow you to craft as many as you want! EQ & EQII In-Game Flags: If you play AQ and/or EQII, you'll have access to the Settler Landmark Flag you can carry or put in your home!
• Founder's Pickaxe: This gives you an upgraded pick combined with a secondary tool, the axe, to start the game with.
• Title: Founder: Unlock the exclusive Founder title for use inside of Landmark.
• Forum Title: Founder: Unlock the exclusive Founder title for use on the Landmark forums.

The Explorer Pack

Includes all items from The Settler Pack

• Explorer Flag Item: An exclusive flag item you can place that proudly shows off your status as an Explorer to everyone! We're even going to include a recipe that will allow you to craft as many as you want! EQ & EQII In-Game Flags: If you play AQ and/or EQII, you'll have access to the Settler Landmark Flag you can carry or put in your home!
• Ring of Bounty: This exclusive ring can be equipped to give you a 10% bonus to all resource gathering when out exploring the world.
• Mega Pocket: Use the Mega Pocket to upgrade either your personal inventory or your vault! A must have for every collector!
• Courtier's Regalia: This exclusive Courtier's Regalia will show everyone just how serious you are about being an Explorer! (Both genders included).
• Tech Sergeant's Gear: Show off to everyone with this exclusive Tech Sergeant's Gear! (Both genders included)

The Trailblazer Pack

Includes all items from The Settler Pack and The Explorer Pack

• Trailblazer Flag Item: An exclusive flag item you can place that proudly shows off your status as a Trailblazer to everyone! We're even going to include a recipe that will allow you to craft as many as you want! EQ & EQII In-Game Flags: If you play AQ and/or EQII, you'll have access to the Settler Landmark Flag you can carry or put in your home!
• Mastercraft Bracer: This exclusive bracer gives you a bonus to crafting, making it more likely the creation will be a high quality item!
• Void Vault: This vault storage has an exclusive look and will give you a distinct advantage by allowing you to use the universal vault network without having to craft a personal vault!
• Noble's Regalia: This exclusive Noble's Regalia will make your friends envious! (Both genders included).
• Tech Commander's Gear: Be the only one of your friends to own this exclusive Tech Commander's Gear! (Both genders included)
• 4 Shareable Unrestricted Closed Beta Keys: Get four shareable unrestricted keys to the Landmark Closed Beta for your friends!
• Your Name in the Landmark Credits: Your name in lights! Okay, more accurately, in pixels. Immortalize yourself by being part of the credits for Landmark!
• Head Start Open Beta Access: When we wipe between Closed and Open Beta, everyone will lose their claims. As a special perk, Trailblazers will get a 48 hour or more head start into Open Beta, ensuring that the have time to find amazing new claims.