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.