An SSD is much better for gaming than an HDD, period.
Traditional hard-disk drives simply cannot compete with the kind of performance that solid-state drives offer, and the gaming industry is ready to move forward.
SSDs are much cheaper than they were only a few years ago, not to mention that even the 9th generation consoles, the PlayStation 5 and the Xbox Series X, will also be using SSDs for storage.
So, you should definitely pick an SSD as your primary system drive if you can afford it. If you’re really pinching pennies and need more storage than what you can get in an SSD that fits your budget, using a smaller SSD as your main drive while also using an HDD for storage is always an option.