L'Avventura or La Dolce Vita

Regarding the OP's question first of all neither are my favorite film of each director. Ftr my favorite Antonioni is L'Eclisse, while for Fellini it is 8&1/2.

Overall I am frankly confused by those here who in effect saw the two men and these two films as very different. They were first of all friends, if not great friends, who respected each other's work and undoubtedly were aware of the other while they were making films. Although Fellini garnered world fame earlier than Antonioni, there was also substantial overlap, and the relevance of this particular OP relates to the two films made at the same time.

Both men were reflecting as well on developments in Italian, and Western, society at the time, from a time that although 55 years ago I think resonates quite a lot today. I think both men tried to see what was around them but in a way that was forward looking; hence that greater resonance than otherwise.

And both films were generally concerned with existential alienation, in hte context of relationships in LDV's case more of a hodgepodge for Marcello's character, in L'Avventura for the sequence for Sandro from Anna to Claudia.

I also do not see one or the other as having a more optimistic ending, despite comments here. For Marcello it is true that the vision of Paola at the end implies despite his turning away to join his alienated/alienating group is and will likely there should he return. Yes, that is an element of optimism. But it is far from clear how and when, if?, Marcello will in fact return. For Claudia at the end, I personally see little hope over time at least for her relationship with Sandro, but in short her problems with Sandro are largely particular to him, and there are other men in the world for her. (To put it another way, a year from the end of each, who will be happier Marcello or Claudia? There is no clear answer.)

But of course there are differences:

Steiner as Marcello's alter ego; doubling. Fellini clearly views Steiner as a comparitor to Marcello. There is no comparable device in L'Avventura.

Setting; Landscape. There are clearly Fellini films that make great use of natural settings, La Notte di Cabiria perhaps foremost coming to mind. But many others. La Dolce Vita is a mostly urban film. L'Avventura obviously uses the large section shortly after the beginning on the water and island to frame that which transpires later, a kind of underlying remembrance following hte characters (along with the memory of Anna). But then there are many urban scenes that follow. Antonioni thus was using a juxtaposition of nature and the urban environment (kind of reminding me of Wordsworth's Lines Written Above Tintern Abbey).

I could go on, and if anyone is interested in this discussion will.

As for hte question itself, though, I could say I prefer whichever I have most recently seen, and in fact have them right next to each other in my list of favorite films all time. Both are great. I suppose I ever so slightly prefer L'Avventura.