Forerunner 70 vs Index Sleep Monitor

You can check the specs on Garmin's website, 

• Forerunner 70 specs
• Index Sleep Monitor specs

The only difference I can see is the additional skin temperature monitoring (which has very limited use, even if you're a woman).

If you don't mind sleeping with your watch on your wrist, there is basically no difference in using one or the other. Apart from all the hassle being introduced by having to switch between them and sync and charge and update two different devices.