I say it depends on what you like to drink.
Go with the Tassimo if you are more of a latte, capachino, espresso, coffee, hot chocolate kind of person.
Go with the Keurig if you are more of a just coffee, tea, hot chocolate kind of person.
I have a Tassimo and we love it. But we like capachinos and lattes more than just plain coffee or tea.
I just bought a Keurig for my ILs for Christmas, but they are more of the tea/plain coffee drinker.
One leg up for Keurig - you can buy a separate filter and "make" your own coffees. So if you don't care for the brand of coffees made for Keurig, you can buy the Starbucks coffee (or whatever suites your fancy) and use the filter to make whatever you want.
And because I was looking for K cups as a stocking stuffer for the ILs - I found both Tassimo brands and Keurig brands at Bed Bath and Beyond, Target, and HEB (a grocery store) as those were the stores I was in.
And you can ALWAYS get whatever you want ordered from either brands' website.
These reviews and comparisons may help you:
http://hubpages.com/hub/Tassimo-Brewer-vs-Keurig
http://www.all-about-coffee-makers.com/one-cup-coffee-maker-reviews.html