which wireless connection is better connecting to an Android Device

Projects General Guidance
1

Which connection is better to use to connect Arduino to Android Device
Wifi Module or Bluetooth Module?

Basically the Arduino will send Heart Rate data to Android
while the Android Device sends commands to the Arduino

2

Personally I would go with bluetooth for your application.

3

+1 for Bluetooth. In my opinion, much easier than WiFi. I use an Android app called Bluetooth Electronics (available in the App Store). There are also several Bluetooth terminal apps there.

4

+1 for bluetooth because WiFi would unnecessarily complicate things. But 1 point to note is that bluetooth modules and WiFi modules are same in cost (HC-06 or ESP12E/F). Here is a cheap bluetooth module https://www.ebay.com/itm/254162945292