Leviathan and the Ultimate Starter Set both put Space Marines and Tyranids on the table, but they are different products on the secondary market.
Leviathan
Launch mega-box. Often sold sealed, as halves, or as individual kits. Completeness and which half you get matter more than the word Leviathan alone.
Ultimate Starter Set
Current intro SKU with terrain and rules components. Sealed retail comps are usually plentiful on Shopify, which makes it a cleaner alert target for new players.
Which should you buy?
New players who want a supported starter path: Ultimate Starter. Collectors chasing OOP launch value or specific halves: Leviathan, with strict photo checks.
Is Leviathan still worth it?
Only at a price that beats buying the kits you actually want separately. Compare live listings on both box pages before you decide.