I have both and love them both. I started w/ a Keurig a few years ago, it broke. I bought another, it broke, and I was annoyed, but my sil told me to call them and they'll send me a new one for free (they make their money from the sale of the coffee more than the coffee maker itself). In the meantime, I bought a Tassimo.
I use my Tassimo more - because they have a French Vanilla flavor coffee I love. When people are over, they tend to use the Keurig (but probably because they're used to it).
Compare/Contrast... the Keurig has to heat up for a couple of minutes when you turn it on (unless you keep it on 24/7 which I know some people who do. I just don't). The Tassimo doesn't require that warm up time. (really no biggie though). The Tassimo seems to make a hotter cup of coffee. The Tassimo can do lattes (with the foaming milk). My dd LOVES hot chocolate from the Tassimo, with the foaming milk.
If I had to pick only one, I'd pick the Tassimo, because of the foaming ability.