When making a website should you choose sitemaps, userflows or both? Are sitemaps used only when navs feature sub navs when hovered?
Sitemaps are a simple visualization of the pages that the website will consist of. I use sitemaps when developing a website for a client so we can agree on the pages and their content.
It's as simple as a square with a word and maybe a small description. Like 'frontpage' with the description 'small introduction'.
It's a simple visualization for the client and for you.
Sitemaps can also be useful when laying out the navigation of the website. The information architecture.
By drawing lines you can visualize the links between pages. Most websites I've build are Flat or Index websites (not like flat design, but flat structure. Here is an interesting read), but when you have a strict hierarchy or multidimensional hierarchy with a lot of pages and only certain ways of accessing those pages, sitemaps can help -not only the client- but especially you make sense of the structure of the website.
User Flows are more reserved for (web)application rather than websites. When there are tasks to be performed by the user, it's smart to first think out the steps the user will take when completing the task.
A simple example is the purchase of an item in a webstore. You'll want the user to be able to purchase an item is as little steps as possible. When creating a webstore without thinking this through beforehand you might stub your toe on poor user experience later on.
In this example, playing around with user flows can help you analyse the steps and perhaps enhance the experience. Creating simple prototype tests will also help in these situations.
Both tools help you understand the website or application you are building and allow you to optimize structure and flow for better user experience. But don't think you're set after using both tools before the development fase. Even after release you might find out that one thing you decided to do because you thought it would be killer for the user experience, isn't that well received with your users. We call that iterating.
I'll leave you with this last link with all kinds of tools similar to sitemaps and user flows (also good for reading up on sitemaps and user flows)