I am wondering what difference the preposition of makes in the following sentences:
It was more of a holiday than a training exercise.
It was more a worry than a pleasure.
Is of optional?
I'd appreciate your help.
Further to my comment against the question, I should say that it's only my opinion that some AmE speakers perceive including (syntactically optional) of to be somehow more "formal".
But I'd certainly consider the charts below good supporting evidence for my (BrE) perspective that if anything it has precisely opposite connotations. Both the AmE corpus results...
...and the BrE results...
...show that of has gained significant traction in recent decades.
The reason I've contrasted it was and it's in those charts is because we can reasonably assume more of the former are in "formal" contexts, and more of the latter are "relaxed conversational" usages. Clearly only the colloquial contracted form actually favours including of in BrE (because at least some of us think it's a bit of a "hypercorrection" to use it in OP's exact context! :)
So noting that the of version has also overtaken the original simpler version in AmE, this might be because they don't share that "dialectal / uneducated" perspective that some Brits do.
Ordinarily, hardly anyone would notice much one way or the other in OP's exact context - but as implied by my whimsical comment above, it would be more than a bit of a mistake to omit the highlighted word in this very sentence.
But it's only idiomatically that it would be a "mistake" to discard of above. There are actually these 2 written instances of a bit more a [noun] without of in Google Books. They're both from the 1850s, before bit was commonly used in such contexts anyway. And I think it's impossible to make a syntactic case for saying that including more and/or whether the context is more X than Y should affect the grammaticality of the usage.
For example, idiomatically, I might say this is something of a self-referential sentence, in that it uses an instance of the very construction OP is asking about. No-one would ever think of discarding of there, even though many wouldn't care much one way or the other if I'd replaced something with more.
For the learner who just wants a reasonable "rule of thumb" here, I'd say the easiest thing is to always include of if all the following examples seem syntactically similar to you...
Just ignore the possibility that some Brits might think you're a bit "rustic". In the long run we'll all be using of like this a lot more.