Rankings are useful in terms of international reputation and name recognition - if you care about that, ETH-Z is probably the better choice.
However, the specific work you'd be doing will naturally be different, and could be better at Delft - which one interests you more?
Apart from that, Switzerland and the Netherlands are very different countries, and you should research the two to see which one fits you better (you won't be working 24/7 after all). Switzerland is good for the outdoors - hiking, etc, but is a bit boring in terms of urban living, unless you are rich and 50 years old. The Netherlands seem to me to be a much friendlier place for young people. I don't know as much about the Netherlands though - I've lived in Switzerland, and only visited the Netherlands.