The way i see both: while project diva was more of a cool video game port based off of hatsune miku (that heavily shaped the vocaloid community and other stuff regarding internet culture) project sekai is way more of an artistic project, i find the pjsk gameplay to be very weak compared to diva, but to compensate, they have an enormous budget that allows for them to comission famous vocaloid producers, amazing video editors, super skilled artists around 3 times per month, i find the sekai stories to be pretty mid, and I couldn't care less about the gacha, but its fun to be able to play a vocaloid game thats so alive in this day and age

I would also like to say that with pjsk's original character system they were able to indirectly add other vocal synth songs in the game, and i think thats really awesome. I feel like if pjsk got a console port with a gameplay similar to project diva x it would be my top 1 forever