I see Chromebooks and tablets as serving different purposes, so to me it doesn't really make a lot of sense to say one is better or worse than the other. Provided you understand the capabilities and limitations of each, both are valid choices in different circumstances.
Chromebooks are good for serious programs such as word processing, spreadsheets and databases where a larger screen, more powerful processor, proper keyboard and a mouse are required - provided of course that everything you want to do can be done online in a web browser. They can also be used for online games, watching streamed video and general web surfing.
Tablets are more portable and they're good for playing games, watching videos and doing a bit of simple web surfing provided hardly any text input is required.