Advantage: H. Sapiens
Seems crazy, but consider that weapons are a force multiplier which lets the basically unarmored person contend with lions and tigers, which are some of the best killing machines made by mother nature. The differences are not extreme enough in comparison to things like logistics, fighting techniques, and overall strategy to matter.
I bet similar wounds bring them down as easily as us. They have no horns, no meaningfully thicker skin, no bone plates... Nothing in the h. erectus body screams "major advantage" over your average h. sapiens and reduced mental capacity is too much a disadvantage.
Reach and Strength Helps Only a Little
As a HEMA practitioner and mostly average man, I have literally crossed swords with people larger and smaller than myself. Advantages in size and strength matter most for grappling, but can be counteracted by technique and speed. The extra height (if any- see question's linked wiki page) and strength may result in stronger blows, but not that much stronger. Some h. sapiens skilled in fighting, either as a unit or individually, can use their weapon(s) just as easily against a h. erectus as a h. sapiens.
Strength, of course, becomes nearly negligible when talking about firearms. Guns and cannon are just so strong and damaging that all members of the homo family would die equally well from these. That's the advantage of not using muscle to power your weapons.
Brains Help A Lot
Using the assumption they are worse at tactics than humans, but not overall strategy and logistics, this gives the advantage to a unit of h.sapiens over h.erectus. Things change when fighting as a unit, which requires discipline and coordination. You need to recognize when your buddy's man is open and strike them when they are preoccupied. Alternatively, knowing when to stand ground and defend the musketeers is more important than making a hole in your line looking for a kill. Ultimately, coordination is more important on the unit level than individual capability.
Assuming h. etectus is worse at logistics and strategy, then they will generally lose to h. sapiens in war. The capacity to stay organized, fed, clothed, and armed does more to ensure victory than individual efforts or a big battle. Being at the right place and time to cut off and weaken an opposing army is why you get generals! This has (and may always) be true throughout human history. Hungry, cold, and/or outnumbered solders don't fight well in spite of their peak ability for violence.