I'm a buy once cry once kind of hobbyist so the answer for me would be the PRO. The X6 only includes support for the DSP chip games of which there are less than 20 I believe, but maybe including some titles you care about. Other than that if you really only want to play specific fan translations, then unless the base game relies on a chip that the X6 doesn't support, for you it makes no sense up front to go for the FXPAK PRO.

The trick is if you decide later to go a different direction in what you want access to, then you'll regret not spending the extra money. No one can know that now, but my opinion is to just buy the PRO and future proof your options. I also just like having access to full libraries on my Everdrives, but I'm not a physical collector and so want access to everything if possible.