I like both pieces of equipment however I find the treadmill better for burning kilojoules than the bike. I have found I burn about 2-3 times as many kilojoules by running on the treadmill than riding the bike. Also, the treadmill is more versatile as you can walk, walk on an incline, jog, run and run on an incline. It is also better for interval training. It gives you more options for starting off low intensity when you're unfit and slowly increasing your intensity as you build up your cardiovascular fitness. Also, because it is weight bearing exercise you will be building up your bone strength as you work out (the bike still helps with bone strength but less so than the treadmill).
On the other hand, the bike would be more appropriate if you need low impact - the treadmill is high impact as you are stressing your knees when you run (there is no stress on your knees when riding the bike). If you have arthritis or knee/hip problems or are very overweight then maybe go for the bike instead to protect your joints. Another low impact option is the cross trainer which simulates running but is low impact because your feet never leave the machine.
