The ayah is speaking to those who deny the Day of Judgement because they don't think they can be created again. Further in the passage, Allah quotes them:
‘What! After we have died and become dust and bones, shall we really be raised up again, along with our forefathers?’
Say, "Yes, and you will be [rendered] contemptible." (37:16-18)
The purpose of the ayah that you quote is to ask a rhetorical question to show that Allah has the power to raise people from the dead, since after all, he created that which are much greater than mere humans.
Then inquire of them, [O Muhammad], "Are they a stronger [or more difficult] creation or those [others] We have created?" [...] (37:11)
So, either interpretation is fine as long as it gets across that Allah is saying that humans are not at all hard for him to create.
Either because He has created those that are stronger than them (stronger as in more magnificent or more enduring). Or because He created those that are more difficult to create hence the difficulty of raising humans from the dead pales in front of Him.
According to Ibn Kathir, it is talking about difficulty to create not strength. And that seems more likely to me as it fits more.
But, either one is possible.
Then, Allah mentions:
[...] Indeed, We created men from sticky clay. (37:11)
To show that they were created from a material that is so insignificant or weak compared with other creatures like angels who were created with light. It might also show that the material is the simplest or easiest of materials to create from compared to light and fire.
A similar verse is in Surah Naziat, where unlike here, Allah compares humans to non-living creations like earth and sky:
Are you a more difficult creation or is the sky? Allah constructed it, raising it high and perfecting it, giving darkness to its night and bringing out its morning brightness.
And the earth, too, He spread out, bringing waters and pastures out of it, and setting firm mountains [in it]. (79:27-32)