In my opinion both are benefitial for technique. One will help you with baroque pieces and maybe indirectly with Mozart and Beethoven who both played the well tempered clavier. The Chopin etudes will prepare you for his pieces and for Liszt's , since he acquired much of his technique by playing them ( Chopin dedicated his op.10 to him because he figured out how to play them).
But there's something the WTC has that the Chopin Etudes don't. The first one will teach you harmony, theory, how to tune a harpsichord or clavichord, and the musical forms of the time. It will teach you how to write fugues,etc.
The Chopin Etudes , though very musical, lack this aspect.
Best wishes,
Gonzalo.
I must respectfully disagree with you about the Etudes not teaching much in the way of harmony, theory etc. The Etudes are immensely informative in these areas imo. And very forward thinking too - Iv heard Op 25 No 11 described as one of the earlier implimentations of polytonality (though I havent analysed the piece to verify this).
At the same time, Bach WTC does not teach you how to write a Fugue. The Fugues may be very good models and demonstrate a wide range of contrapunctal and inventive techniques, but I dont see that it teachers anything. To understand those pieces still requires extensive explanation.
Personally, I would have to say learn both! Make it a long term plan. But if you cant, then understand that the Chopin Etudes will be more applicable to romantic music, as the WTC is to Baroque. So if you're wanting to play Chopin and Liszt in a hurry, then learning the Etudes (if you are able to) will be the order of the day.
There ofcourse is a certain degree of cross over. Indeed the WTC is considered by many to be great for technical development across the board. But if time of the essence, then I guess you have to choose the most relevant.
SJ