So, I didn't share this earlier, but only because I was pressed for time on my (quippy) response. But a friend and me often fought each other using (rather shoddy) recreations of antique weapons during our high school days. We had wooden gladii, long and short spears, disc shields, and also a small array of other more ridiculous things (nets and chains and whatnot; basically anything we saw in Gladiator or the original Spartacus movie). Doing one-on-one fighting, I found overhand and underhanded grips to be similarly effective, but very dependent on the situation. However, I only found them to be similarly effective because of the fact that we were fighting one-to-one. Thinking back on how I used the spear alone, or the spear and shield together, I would say with certainty that in formation overhand would be far superior. I know it was more maneuverable that way, had more force in thrusting, and was easier to return to attack position when batted away. The one scenario where I remember underhand being (possibly) better was when the shield being used was very small. In that case, holding the shield above the spear allowed one to guard one's center while thrusting, and also use the shield hand to partially guide the thrust. It was a small advantage gained by underhand, but it was there. That being said, it was only in one-on-one combat with a small shield that such tactics proved helpful. In general, I would always use overhand, largely for the reasons Rad gave above.
P.s. For anyone interested in hands-on research, I would love to again do some spear and shield battling! I challenge you all!