Bluetooth or WiFi is better for connecting multiple devices to 1 master device?

I am creating a new Android Application and I stuck in an point where I cannot decide which technology has to use.

There is a master device and there are a lot of slave devices (more than 10). All slave devices need to connect master device and need to sync with its database.

all devices should be synced simultaneously

I have few questions.

1. What is the maximum devices can be connected to a device via bluetooth and WiFi.

2. Both bluetooth and WiFi allow to sync simultaneously ?

3. Which one is faster while syncing ?

I have some knowledge about its speed and battery life. but No clear idea about those 3 points.

Please give me your advice to select best one.

Thank you