Which version of Xak came first? MSX or PC88?

That information should be correct, generally most computer games back then in Japan were made for PC-88 series first then released on others (and later on, some games appeared first on PC-98 as 98 got more popular).
I had a MSX/2 as well as a 88 and a 98. As a static image, graphics look better on the 88/98 as they had higher resolution (88 had max resolution of 640*200 and 98 had 640*400) than MSXs. Although they only had 8 (88) and 16 (98) colours. But the game play was not better than on MSX. Characters did not move smoothly, and scrolling was usually wavy. So action/shooting games were not good on PC-88/98, and that's probably why there were many RPGs (some with part action) and adventure games.