My reasons for favoring the Cello are the larger fingerboard, and tone. I'm a bassist so I can relate to the Cello more. It's also easy to play upright as opposed to holding a Viola or Violin onto my shoulder with my chin. Speaking of chin rests and shoulder rests, I think I'm going with Artino magic pads on the top and back of my Viola, replacing both rests. It's very comfortable for me, I use the 3.5" and they're very affordable!
Holding a violin/viola is certainly more awkward, and probably contributes to there being a lot more successful adult beginner cellists than adult beginner violinists or violists.
It's quite common in community orchestras for as many as half of the cellists to have started as adults, while only two or three people in the combined violin and viola sections of the same orchestra started older than 12.
I played a cello for 5 minutes when I was 16 or so. It was easy to get a good tone out of it.
But there's a twoset video of them learning cello in an hour, and the intonation doesn't come easy!