Is it better to use whole (full-fat) or semi/skimmed milk for a cappuccino, or does it make no difference (other than the taste)?
Will one type of milk be easier to froth?
The short answer is yes. The absence of the milk fats means that the foam will not "coagulate" as easily.
The proteins chains in milk are polar: one end of the chain is hydrophilic (attracted to water), and the other is hydrophobic (repelled by water). Because milk is mostly made up of water, as soon as those proteins unfold, exposing their ends, the hydrophobic ends immediately try to get as far away from that water as possible. If you were to look at a single tiny bubble in a cup of foamed milk, you'd see that the hydrophobic ends of the milk proteins are all pointed inwards, towards the water-free interior of the bubble, while the hydrophilic ends stay put in the aqueous environment the bubbles are suspended in.
This structure helps keep the air bubbles intact for a long time after the steaming > process, all the way into your cup (and into your happy, soon-to-be caffeinated mouth).
For a wonderful article on the topic see:
From experience it seems slightly easier and is less runny. The difference for me is pretty negligible with semi-skimmed vs. whole milk. Skimmed on the other hand always ends up quite watery for me, but that may have something to do with the way I'm frothing it.
Skim milk is easier to froth and it is drier. Whole milk takes longer, but makes for a creamier froth. I think it would be worth experimenting with to see which one you prefer. I like the texture of the whole milk froth better, but its not what I like to have on hand for everything else.