WiFi Direct vs Bluetooth
If you device is battery powered and you don't need high speed, this sounds like a Bluetooth application. If you use Bluetooth Low Energy (BLE) the drawbacks are shorter range (50-100m vs. 200m) and that Bluetooth 4.0 BLE isn't backward compatible.

A good comparison can be seen here:

Comparison of BLE and WiFi-Direct

Also there are dual chip containing BLE and classical bluetooth:

See this post for dual chip

The future is hard to predict by looking at the past :P