Psst. Get a keg setup, build or buy a carbonator cap, and your soda water will be cheaper for it, and you can use it to force carb small quantities of beer (good for taste tests) or force carb a full keg. You can then get a kegerator/keezer setup, add faucets and dispense beer!
Really though, if you're even considering any sort of carbonator, go for the co2 tank and carbonator cap. The investment of material is the same cost as a sodastream, plus more versatile.
However... if there are people around who are non-technical...or whatever the appropriate term for scared of the co2 tank regulator adjustments, tank valves and connecting the ball lock. (Like kids, some girlfriends, some mothers) then get the sodastream (and then say it's for them and get the kegging setup!)
Compare this: http://stores.kegconnection.com/Detail.bok?no=247
with this: http://www.bedbathandbeyond.com/product.asp?SKU=131624
Both have the same cost structure for initial equipment. (120 vs 99-150 depending on sodastream)
The sodastream unit is only capable of 60 liters per mini tank which holds 14.5oz co2.
The small co2 tank weighing 5 pounds, (in accordance with 60 liters/14.5oz * (5lbs*16oz/lb)) is expected to give over 300 liters. A sodastream tank "refill" or exchange costs $15 per tank (usually $30 for two tanks) and a 5 pound refill will cost around $20.
Force carbing beer this way does cause some foam, but you can let it rest for a day and it'll be fine.