Having been around for years now, Bluetooth technology has undergone a lot of changes. Currently many devices support various version releases making it imperative to know whether the Bluetooth headset you're considering is compatible with your device, if you're one of those wants to avail all the features offered by that version. If all you're concerned with is your device connecting without any issue then then you could stop reading here or read further to know what you might be missing out on.
The latest version of Bluetooth will always be compatible with the version that existed before it. For eg: a Bluetooth headphone supporting Version 4.0 will work with a mobile having Bluetooth version of 3.0.
Bluetooth 2.0 doesn't have features like voice recognition and it sucks in more power, making it difficult to use for longer periods. Version 3.0 and higher rave of Bluetooth range of upto 100 meters where as 2.0 goes upto only 10
Version 3.1 is fast but the 4.0 is faster and also uses less power in transmitting data even it the device is 50 meters away. Both support multi point connections. however connections wit 4.0 are more secure.
Always go for the last version: it is more reliable, faster and usually more power-friendly.