It really depends on what you want to do. The museum pass is not good value unless you are going to do the tourist sights. If you are doing the tourist sights, then it is probably the way to go. Otherwise, then your best bet is probably a week card. If you are around for 3 days, you may need to buy 4 day cards, in which case the week card is better. You almost certainly only need to cover zones A and B - you'll only go into zone C if you are going way out - the only place there that may be of interest is Potsdam.
The Welcome Card looks like a normal card with tour guide stuff thrown in. You can probably pick up most of the stuff in it for free anyway, and I guess you will be coming with your own Berlin Tour Guide.
Have a look at this link for an older discussion on this topic.
http://www.tripadvisor.com/ShowTopic-g1873...ard-Berlin.htmlHope this helps and have a fun time - Berlin is a great city - make sure to also take time to hang out with real folks in pubs/cafes, and if around at the weekend, check out one of the markets.
Colin