My apologies if I'm wrong, but isn't this beyond the scope of this sub?

While I do encourage you to follow the mods' direction to use r/askanthropology, I would like to add a small fact that may help you formulate your question in that subreddit:

What sort of upper body strength do you intend to ask about? The biomechanics of strength depend upon the particular action, and the neanderthal did not have identical physical construction to the human frame.

It is, for instance, widely believed (as of 2013, the last time I was current with this research) that Neanderthals' shoulder architecture prohibited strong long-distance overarm throwing, such as thrown spears. In a contest of throwing, humans would definitely be able to exert more force. However, in an impactful charge with that same spear, a Neanderthal's superior muscle mass exert more force than a human would. According to the research of Maria Mednikova, Neanderthals mostly hunted large game up close. While I don't have access to her paper on this right now, her abstract and comments make it clear that for this they had superior upper body strength and mass to normal humans. This would be necessary for hunting large game with a non-thrown spear on foot- highly dangerous, highly physical!

So I wouldn't want to be punched by one, but in a baseball game I would pick the modern Human. If you are asking about lifting weights or pushing heavy objects I cannot speculate as I have only seen research on differences in weaponry use for hunting.

Source on throwing: Rhodes, Jill A., and Steven E. Churchill. 2008. “Throwing in the Middle and Upper Paleolithic: Inferences from an analysis of humeral retroversion.”

Edit: As I see you have already asked this question on the anthropology subreddit I have copied my reply to that. Mods may direct me to delete this post if it is now obsolete, but perhaps someone who wouldn't see it on that subreddit would be helped by it here.