I had GP during a few months and did use it to mostly play some old MS games I had pending. I prefer PS+ because its catalog is way bigger and has many more appealing games to me. I am not interested on most day one GP titles, or at least not enough to play them day one, because for me they are B/C tier games and the little time I have to play I use it in games that for me are A/S tier games, either new or old.
As an example, recently I subbed to PS premium again and had there way more Sony games I wanted to play but didn't buy back then like Horizon Forbidden West, Returnal or Demon's Souls remake. Plus some retro games they added in recent months like Ridge Raccer Type 4 (PS1) or Ridge Racers 2 (PSP) plus some interesting demos of games I wanted o try.
This last year I didn't play as much as did other years so I even have been most of the year without PS+, but in previous years I did buy many games and with the discounts from PS+ I saved more money than what a sub did cost to me the yearly sub. So with the discounts alone it was worth for me.
Now a year of PS+ Premium is like $150 bucks, I'm not sure if this year I'll use more than that in discounts, but make sure a good chunk of it will be compensated. So for me, the real price of the game sub will be way lower.
The MS discounts instead doesn't affect me because I don't have an Xbox console and I don't buy on PC stuff on MS's store, but in Steam instead.