If I think really hard I tend to feel 天下 emphasizes the physical geographic range and 地上 is more about being conceptually humanly or worldly, ie only excluding what is "out of this world". For one, 地上最強 is usually more frequent than 天下最強, but I'm unsure if they really differ in that way.
- literally "under the (same) sky", "under the sun", or "the whole world insofar as 天 extends"
- since 天 also has "world order" meaning, what people using the word are concerned is virtually "the whole world we know" or "wherever our society spreads out"
- it's basically a secular idea about the visible world that isn't associated with any otherworldly imagination
- outside fantasy worlds, the word has been reserved for East Asian regions; the original usage was about the world in whose center lies China, but at least Japanese and Vietnamese tend to apply it to their own territories
- literally "overground", "surface", or "(all) on the earth"
- could be a synonym of "world" (= 世界), but also likely to be confined to its literal sense: earthly world as opposed to heavenly or nether world, or all over the land as opposed to sky or sea
- of course the word has much more mundane meanings that English "ground" or "surface" has
地上最強 is usually more frequent than 天下最強
It's an interesting point. Indeed we often use 最～ with 地上 but hardly with 天下, though we tend to combine "incomparable" with 天下 (天下一, 天下無双 etc.), which in turn rarer with 地上. Maybe it has something to do with the nuances that 天下 contains some implicit hierarchical order, but 地上 just looks around the world plainly.