I have owned both. Each one has it's advantages and disadvantages.
Tassimo:
I have it now. It's far more reliable and I believe it makes a much better cup of coffee. The Tassimo has a barcode reader inside, which reads a barcode from the T-Disc and adjusts the brewing cycle accordingly (temperature, pressure, quantity of water, etc.) It's pretty intelligent and works remarkably well. Friends and family cannot tell the different between the espresso made with Tassimo and one made with an espresso machine. With the Tassimo you can make Cappuccinos, Lates, etc.
Now, the downside is the availability of T-Disc. There are relatively few brands represented here and just about all of them are high end European coffee houses (except for the dreaded Maxell House). Starbucks stopped supplying T-Discs (huge reason why I bought it, but they did replace it with good quality Gevalia coffee). The coffee is generally very good. You can of course order all of them online. Overall, T-Discs are more expensive.
Keurig:
I used to have it until it broke a month outside of the warranty period. $240 down the drain. A had the smaller one-cup Keurig which also broke fairly quickly. The brewing technology is very simple and is basically limited to the quantity of water used to brew the cup (large, medium, small). Every cup gets the same treatment, which honestly isn't always the best way to brew coffee. You cannot make Cappuccinos, Lates, or Espresso. Only a straight up cup of coffee. On the plus side, coffee is cheap and widely available.
So, to sum up:
Tassimo:
- Better coffee, more drink type choices.
- More reliable hardware (made by Bosch)
- Availability of coffee limited (few stores carry it)
- T-Discs more expensive
- No more Starbucks
Keurig:
- Good coffee, limited to simple cup.
- Less reliable hardware
- Availability is abundant (few stores don't carry it)
- K-cups are less expensive
- They have Starbucks!
So, it maybe hard to decide. Honestly, I'd go for a nice Tassimo and a small portable and cheap "Keurig" machine made by Hamilton. Best of both worlds.