Both selenium and etl testing has it’s own importance.if you want to perform front end automation then selenium is best open source tool now a days in market and future of selenium is also good.but if you are interested in doing data warehouse testing then go for ETL testing.

But looking at the current trend in market , my personal suggestion would be to go for selenium testing because demand of selenium tester is increasing becoming high.if you know any language like Java, python or c# then it would be easier for you to learn selenium.

Hope this helps you to decide!

Also Visit: Selenium Training in Pune