You branching off an existing duct work trunk line and you come to a "fork" in the road. Use a "Tee" or use a "Wye"?
Here are some things to have in mind.
Standard Y Branch:
Sometimes spelled out "wye branch". Usually used for supply air duct runs. This means the air is being forced into a space. The 45 degree tap in the direction of the airflow is best for this application. There is less resistance (air friction) because the air is being "steered" through the turn instead of stopped and redirected like it would with a T.
Tee Wye: These are usually used in exhausting applications. This means the air is being sucked out of a space. Think dust collection systems, paint spray booths, greenhouses, even bathroom exhaust. Using a regular wye branch (above) for exhausting creates extra friction. Also the right angle of the Tee Wye makes it easier to attach hoses to and can be great for tight spaces.
Splitter Wye: These are going to provide optimal air flow if it is the final, or only, Y branch in the run. The closest you're going to get to splitting the air 50/50 using standard fittings.
Bullnose Wye: Very similar tot he standard wye branch. I don't know if they afford any advantages in terms of air flow. I like them because they feel more sturdy than standard Y branches, and sometimes they're cheaper. Technically, they're probably going to "buzz" a little more than a standard wye, but probably not enough to make a difference in typical home applications.
Feel free to email me or comment if you have something to add. Until then, get out there and D.I.Wye.