My guess would be Widmung is easier. Not necessarily because it is technically easier, but it's significantly shorter and also less complex in structure and texture, so not as difficult to develop an understanding for.
Having said that, I think both are musically advanced pieces that could fit into a conservatoire student's end of year or even graduation recital programme, as part of a diverse collection of music. OK, neither is of impossible difficulty (for example like the Rigoletto Paraphrase or 'Mazeppa') but still...
Dunno exactly what kind of diploma system you are referring to, if you want to compare it to a piece that you;d probably be able to find back in any repertoire lists as a reference I'd say the required skill is comparable to Chopin's op. 25-1, the 'Aeolian Harp' or perhaps op. 10-3 'Tristesse'. Hope this is of some use for you.