According to my helpful grammar the word ‘plenus’ is accompanied with an ablative or a genitive.
Which is used when?
With plenus the so-called partitive genitive is generally used, which corresponds nicely to the english “full of…”.
It can, however, as you note, take the ablative. This is more common in verse than prose and has the meaning more “abounding in…”, so, one must presume, refers to a lesser state of fullness (although I wouldn’t read too much into that).
Generally use the genitive.
~dave
They are identical in meaning, owing largely no doubt to the same semantic gray area that saw the ablative absorbed by the genitive in Greek. The difference is largely stylistic, and the genitive is more common in Ciceronian Latin than elsewhere (within this period both are about equally distributed).
The ablative in question is one of means or instrument, which points back to the Indo-European instrumental case (extant in Sanskrit). This denotes that with or by which a thing is done. (The ablative also absorbed the locative case, but that’s another story.)
The basic sense of the genitive case was origin, the ablative separation, so the lines are easily blurred (think: from America vs. of America; filled with water vs. full of water). Here the semantic distinction with the genitive is in a sense the source of its filling, but it’s an extended use of the genitive.
There’s a similar phenomenon in Greek. I’m thinking of an ode of Bachylides in which a word denoting fullness takes a genitive and a dative within two or three lines. This is because the instrumental case was absorbed by the dative in Greek.
Nice explanation 