I have both a Negaflat and a glass carrier, but I'm not selling either.
I'd highly recommend the Negaflat if your goal is simply to keep 4x5 film flat. These are easy to find, but it seems the older ones are slightly different than the newer ones.
The reason I keep my 4x5 glass carrier is mainly so that I have a glass carrier when I want one for smaller negatives. For example, I also shoot 16mm and Minox film, and this allows me to make 8x10 "contact" sheets from the smaller negative -- even though the negatives are in the glass carrier in the enlarger, and NOT actually in contact with the paper.