Need help in booking Bernina Express

I am trying to book two tickets in Bernina Express. Need help in understanding the following
1. Is it better to buy a 2 days pass and use that as a ticket or should i simply buy the tickets without bothering for the pass
2. Where can I buy the tickets from? There are too many links that spring up (eg viator etc) when i search for the bernina express but i am unable to figure out which one is the direct site to book so that I get a good reate.

Regards,
Gaurav

I don't really know the various sites, but I would be inclined to recommend the official SBB rail system site:
http://www.sbb.ch/en/home.html

1. Do you just want to travel lby train over the Bernina Pass (Tirano - St. Moritz) or do you want to book a seat in one of the fully airconditioned panoramic Bernina Express cars?
2. What do you mean with 2 days pass?
3. May be you have a look at http://www.myswitzerland.com/en/home.html http://www.swisstravelsystem.ch/en/ http://fahrplan.sbb.ch/bin/query.exe/en http://www.rhb.ch/index http://www.rhb.ch/The-World-Heritage.1070.0.html?&L=4 http://www.rhb.ch/Video-Bernina-eng.782.0.html?&L=4 http://www.rhb.ch/UNESCO-Clip-eng.1061.0.html?&L=4

About the two day-Swisspass; it might be not that great; you'll have to compare:

The normal ticket is CHF 184.-- in second class and CHF 304.-- in First Class for the above trip. And you'll have to book the bus and the Bernina Express in advance (Bus leaves Lugano at 10 am and you'll be back in Zurich around 8 pm).

Booking pages are the official:

https://www.sbb.ch/ticketshop/b2c/sprache.do?en

or

http://www.rhb.ch/Ticket-Shop.1103.0.html?&L=4