As someone who develops for both, I would say its easier to customize C5 as the override architecture is really easy to pick up, you can customize core blocks very quickly and there is no limits to the number of custom templates you can add to a block, for example, auto nav, you could have your main navigation, a sidebar nav, a footer nav, a breadcrumb, etc the list goes on, all with custom markup, but using the functionality of one block. In Wordpress to do the same, you are looking at multiple walker nav classes, and if you have ever built custom nav using a walker you will know what a pain that is, and don't get me started on building Gutenberg blocks.

Theming and block building is also fairly straight forward, I would say C5 is a touch more difficult to theme than WP, and plugins vs blocks are comparable.

The vast majority of clients I have on C5 are happy and I seldom hear from them, the support burden from WP based clients is higher, but not much to be fair. I find people pick up using C5 quite quickly as you edit the page you are on from the front end, WP is a little more detached and people don't seem to pick it up as fast in my experience, training tends to take less time on C5 which is a definite bonus.

As far as speed goes, hosting is pretty important with C5 you can't really stick it on $1 a month hosting and expect it to perform. On a like for like install WP with no plugins, and C5 with no add-ons I would say WP is slightly faster, but I find with WP I rack up plugin use far more than I do with C5, C5 comes with a lot of useful blocks straight off the bat.

The user base is obviously going to be skewed well and truly in favour of WordPress, WP resources are spread far and wide, while C5 pretty much revolves around C5.org, github and Slack, between those 3 places you can usually find what you are looking for.