Modern desktop platforms tend to favor memory configurations with fewer memory modules. When comparing a 2x32GB memory kit and a 4x16GB memory kit, they both offer the same total capacity, however, a 2x32GB memory kit may have options for faster speed ratings than a 4x16GB memory kit.
If you’re building a small form-factor system, your motherboard options are likely to be limited to models featuring only two slots. In such a case, you’ll need to pick a 2x32GB kit if your goal is to have 64GB of total system memory for your system.
A traditional ATX system will usually be built with a motherboard featuring four slots for memory, making a 4x16GB memory kit a viable option for a similar memory capacity spread across more modules (and fill up all available slots for a more "complete" look if you value aesthetics).
If you’re building a system that supports a quad-channel memory configuration, a 4x16GB memory kit would be the way to go. That said, quad-channel must be explicitly supported by your chosen CPU and motherboard combination.
Ultimately, it’s up to your chosen platform, system needs, and preferences. For additional assistance picking a compatible memory kit for your next build, try our RAM & Motherboard Compatibility Checker which will help you find compatible memory kits for your system or our PC Builder which will help you pick compatible components across your entire build!