great question,
For Nike, the issues are:
1. The Nike+ does not have the digital tag feature so there is no emergency information available.
2. The Nike+ is much more expensive. If you already own one then you may benefit from their mileage counting but if you do not you can get this functions (plus the digital health tag) for significantly less money.
3. The MilestonePod offers an on the shoe display for constant update and reminders. Also, with Nike you have to run with a smartphone, our device is self contained.
4. The Nike+ is a Nike product built primarily for Nike shoes. It can work with other shoes but you need to attach it to the shoe in a way that it was not meant to be used and I have heard it impact accuracy.
