Essentially this 10 vs 25 world first debate is silly. Getting world first in either format is IMO equally impressive, but you can't really compare the two formats because they can never be perfectly equal.
Overall there are 3 world firsts:
a) World first 10 man boss kill.
b) World first 25 man boss kill.
c) World first boss kill.
a) and b) should be considered equally difficult feats. c) is more an academic talking point more than anything else.
Technically, I expect that the 25 man race has a higher median skill - ie less competitors, but in general those competitors are stronger. On the other hand the 10 man race has a lot more competitors - but there are a lot more weak competitors. If you looked purely at the top 100 10 and 25 guilds and compared skill though, I am not there would be a meaningful difference.