This is in some ways a pointless question as it is of course both: the OED allows us both. I do however have significant concerns over how ‘learning’ gets used as a noun in the field of education.
Where learning is used as a verb, it always shifts the focus to the object being learnt. One is always learning something and where we use ‘learning’ as a verb, we face the inexorable pull towards the very purpose of our teaching: that thing that we think it is important for children to learn.
Yet when learning is used as a noun, we lose this pull. Learning itself becomes the object. We hear people talk of ‘her learning’ as if ‘learning’ itself were some thing that had an existence of its own. More worryingly, using ‘learning’ as a noun makes it easier to have a conversation about education in which no attention is paid to what is being learnt. This neat little trick is exacerbated when we begin putting adjectives in front of the noun: suddenly we’re having complex conversations about ‘philosophical learning’ or ‘mathematical learning’ which distracts us further and further away from the very thing being learnt.
In short, treating the word ‘learning’ as a noun rather than a verb has the semantic effect of turning a process into a product: the means of coming to know something becomes the end itself. It is indicative of the modern obsession with prioritising pedagogy over curriculum.
The same phenomenon can be found with a whole range of verbs: reading, writing and speaking all get this treatment on a fairly regular basis, though perhaps the worst sins have been committed in the nominalisation of ‘thinking’, where people obsess over what kinds of thinking children are doing without much care for the thing about which they are thinking.
As ever, we need to spend more time getting back to the things themselves: what pupils learn is what matters.