Nobody is using any ammo called HIMARS, that is a vehicle, the 270s use the same ammo as the HIMARS vehicle, 83km range, 250 lbs warhead..
The MARS ii is a upgrade of the 270, faster and more accurate, but 83km range, like all Mars, hi on something or not, but they deliver 12 rockets.
One MARS II = two HIMARS, they deliver the same ammo and they are both very mobile.
The US delivered 22 HIMARS, which is 11 270MARS, concerning fire power till reload.
Just to put thinks into a perspective.
But the HIMARS and the MARS have made a big change in this war, same for all the 155mm with PGM ammo have done. The difference, you fire and you hit, shoot your rockets or grenades and move, get out of the counter fire zone. With PGM ammo you can do fast and precise counter battery fire and the point, you hit.
Don't have to shoot a pattern, which opens you up to counter fire.