Hi Julie! Well, since no one’s responded yet, I thought I’d offer my humble opinion. I have had some limited experience with both merino and alpaca yarns (not the ones you mentioned, unfortunately), and I find both incredibly soft.
Some other things to consider besides softness:
Since alpaca is warmer than wool, you might want to go for alpaca if your Mom lives in a colder climate.
Alpaca is supposed to be less springy than wool, so alpaca mittens may not “hug” her hands as well wool ones. (I can attest that the alpaca hat I made does seem to be quite a bit looser fitting than the wool hats I’ve made, which I find to be rather annoying.)
Alpaca is supposed to be less pilly than merino. Since they’re worn on the hands, mittens are probably likely to experience quite a bit of friction, which may exacerbate any pilling tendencies. (I don’t know how pilly malabrigo is so that may or may not be a problem.) Supposedly pilling goes away once the shorter fibers have worked their way out of the knitted fabric but I don’t know how long that typically takes.
I think that if I ever decided to try making mittens, I’d probably look for a superwash merino because I like the idea of easy care machine washing, and the pilling problem would be moot.
BTW, there is a great article on yarn substitutions here. The author emphasizes not only the importance of gauge, but spin and fiber composition as well. I never knew there were so many things to think about until I read that article!