As a Recon, if you want to use any form of enemy detection you can only equip one of these two options. But which one to choose?
The T-UGS are very small and relatively simple. You can place them down in a more camouflaged spot without them getting noticed. T-UGS will beep when an enemy is moving in range, but you can easily bypass them if you are crouching or proning. In very crowded places this a good to quickly deploy and get many assists. The size of the T-UGS can possibly cause it to last longer. This makes it hard to shoot and destroy, unless a grenade is thrown.
The MAV does basically the same thing the T-UGS do. However it has the benefit of flight, and while in flight the camera has a thermal optic view, allowing you to manually spot enemies, or explosives. The MAV does have a motion sensor , but is much bigger than T-UGS and can be easily spotted and destroyed. That being said, the MAV can be flown high enough to be out of sight, and still used as a motion sensor. The MAV can be used tactically as well, by scoping out enemy areas before charging in to them. Or to search a map for a well hidden sniper. The bonus to the MAV is that it also has the ability to roadkill someone.
For a roadkill you must fly directly into a soldier. Depending on how hard the impact is, or the area you have hit, the MAV will explode. It is still possible to get a small kill streak with the MAV but it’s mostly just a matter of luck.
Both the MAV and T-UGS can be replenished with ammo. Both can also be picked up and repositioned. If, however you are killed and revived, your gadgets will remain, but you will no longer be able to pick them up. they will also self destruct upon respawning.
The choice is yours when choosing, but consider you surroundings or the main use for which you want to use it.