As usual, it depends:

Splendor pros:

• Plays 2-4 (vs strictly 2 for Duel)
• Simpler rules, easier to teach
• Larger chips, better tactility
• Expansion available

Splendor Duel pros:

• Rules and components optimized for 2p
• Nicer card art (subjective)
• More decisions, more interesting draft, multiple win conditions
• Much smaller box, reasonably sized for the game (unlike the original)

If you plan on playing with more than 2 then original Splendor is the obvious choice. It's still a decent choice even if you will only ever play at 2 but value the simplicity and purity of the original design.

Duel is a more interesting game thanks to the new draft and multiple win conditions. "More" and "more interesting" is not automatically better under all circumstances though.

Personally I am keeping both even though we almost always play at 2. We play Duel more but sometimes I just want to play regular Splendor and Duel is not a 1-1 replacement.

The Duel box conveniently fits inside the original's box (insert tossed). I am no longer planning to get the Cities of Splendor expansion thoguh, Duel is filling the "Splendor with more stuff" space nicely.

For 7 Wonders it's more clear cut, Duel is a much better option for 2p.

For Agricola the two games are almost too different to compare. All Creatures (big box) is great in its own way but doesn't feel like a replacement for the original.