It really probably depends on your risk tolerance and what you think about the deal. A lot of people are saying Microsoft will offer 35 and the institutional investors will force it on the board. Alternatively, if the deal doesn't go through, it will likely drop back to 20 as people have mentioned. Microsoft isn't going to trade 30% one way or the other in the short-term, so it's a safer bet if you don't want to gamble or you don't think that the deal will go through. Microsoft stock would be one of the better plays in tech given it's current valuation (though I think tech isn't going anywhere short-term). I'd definately disagree that Microsoft is too big to profit from. Sure you aren't going to turn 300% in 6 months, but for example if you had Microsoft for all of 2007, you'd have made something like 15%, which isn't bad, though I'll admit you'd have also given back all those gains in 2008 if you kept holding.
Interestingly, your second question assumes that Microsoft does buy Yahoo and that Microsoft stays strong afterwards. If you have a strong belief that will happen, then you definately want Yahoo stock, because you'll get a big boost when the deal happens, and you will probably also get Microsoft shares as a result of the deal.