Talking about ETL testing, it is a mix of shell/batch scripting and database testing, you need to know a couple of things before starting with ETL testing, for example, Linux, SQL and if not one, couple of Dataware warehouse tools.
On the other hand, Selenium testing is Java/Ruby/Py/PHP and selenium API on top of it if you know the project configuration tool then its a plus for you.
But both are interesting and challenging based on your likeness towards specific things.
If you like dealing with database and hands-on SQL then go with ETL testing or else go with Java/Selenium. If you also want to learn Selenium you must take up the following Selenium Training Course. If you like reading the stuff you must read the following Selenium Tutorial. Here is a Selenium video tutorial you must have a look at this video. .
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