Wi-Fi vs Bluetooth communications to android phone
Bluetooth (BT) will be overall more reliable. The android device can connect to more than one Bluetooth device but usually only one WiFi device and this you likely want to be a device providing Internet access such as a WiFi Access Point (AP). The Android device will "randomly" connect to more advantageous WiFi signals in its vicinity whereas with Bluetooth it will reliably maintain its connection with any authorized Bluetooth peripheral. A WiFi peripheral will require direct interaction to get it setup on a WiFi AP whereas a BT peripheral can pretty much be turned on, connected to and used straight away.

For an IoT (Internet of Things) sensor device that doesn't need to do much of its own processing or interaction with the RoW (rest of world), Bluetooth is the way to go.