I faced this same dilemma a number of years ago. I picked up a game used William Harridge ball for a nominal amount (about $10) on a trip to Chicago.
I've always had a fascination for fastball speed records. Prior to the modern radar guns, Bob Feller had held the mantel for having the fastest recorded fastball.
His fastball was measured first in a contest where he threw a ball through a paper target easily beating a speeding motorcycle moving at a known speed of 80 MPH. Not too scientific, but fun to watch and record.
In 1946 he has clocked at 98 MPH (or using today's standard of fastball speed measurement, 107.9 MPH) using the Army Ordinance Departments Sky Screen Chronograph.
Not having much into the Harridge ball, I decided to have Feller sign it for me with the 107.9 MPH inscription perhaps 10 years ago. I thought it would make a cool conversation piece if nothing else.
As you can see from my pictures, the BIC blue ballpoint pen sig that Mr. Feller applied has held up extremely well in spite of the ball being old and game used.
But, that being said, the leather used in the manufacture of baseballs is very unpredictable. You never know how the sig will hold up over time. I would suggest that no matter what ball you choose, use a BLUE BIC ball point pen as I have found these to be the best as far as the sigs holding up so well. Never use black ballpoint pen, as these sigs frequently fade like invisible ink FWIW.
No matter what you choose, good luck with your project!
