Hi Devendra,
I'm afraid you won't get an easy, straight answer here. It all depends on preferences and future plans you might have.
First of all, you have to keep in mind that while the tuition is next to nothing at ETH, the cost of living in Switzerland is among the highest worldwide, especially for students in Zurich. Depending on which nationality you have (especially if you are not a EU or EFTA citizen) you might also be prohibited to work for a certain period after you start your studies. Even if you find a job (which might not be that easy if you don't speak German) you will only be allowed to work at most 15 hours per week on a student visa. This would allow you to cover some of your expenses, but probably not all of them.
On the other hand, as you have mentioned, ETH is certainly better ranked, with most international rankings placing it in the Top10 worldwide. Especially if you are planning on doing your PhD in Europe (or Switzerland and Germany in particular) then choosing ETH would be a very good choice. If you plan on doing your PhD in the US then the situation is not so clear. What I found is that a lot of people in the US know the national Top30 or so, but not really any international ones apart from Oxbridge. Maybe I got a biased sample there, though.
As far as I know, there is no separate department of statistics, but only the department of mathematics. The people I got to know while working there were all very competent and pleasant. I'm definitely biased here, but I would say the ranking is well deserved. What I found while doing my masters here (physics though) was that especially the teaching is really excellent at ETH. You have an unusally wide range of lectures on special topics to choose from and most of them are quite good.
Lastly, if you are planning on staying for a PhD ETH might be a really good choice, since the salaries are quite good compared to international standards.
Best of luck, cheers,
Michael