I voted slabbed.
Slabbing helps to reinforce authenticity, and help establish a recognized value vs condition.
Works best with American collectors with U.S. coins, especially if you are happy to leave grading to the TPGraders.
I much rather prefer to grade my own coins.
I have about 200 U.S. coins, some individuals valued in the hundreds of dollars each.
None of my coins are slabbed.
Why?
The U.S. coins form only a part of a much bigger collection of World coins, ancient to modern.
Therefore, all of the coins in my collection need to be fully attributed, with amount paid and date acquired.
Can't do that with slabs.
As a result, almost all of my coins have to be in Mylar / cardboard 2x2's.
I didn't vote this way.
Exception noted below.**
**It is possible to considerably enhance the protection offered by a cardboard / Mylar 2x2. I staple a coin that needs a higher level of protection between two 2x2 sheets of clear acetate plastic, recovered from used food containers.
Thus protected, the coin then goes into a standard 2x2.
Different coins and collectors have different storage / display needs.