There are so many threads on that subject if you google slant vs straight cab.
There seems to be contradicting opinions and in the end it goes back to what sounds best to you.
The biggest difference is more about sound projection than tone itself.
Stage situation miked or not vs rehearsal or bed room.
Cab inner construction (rear loaded vs front loaded) and speakers used will have more effect on tone than being simply straight or slant.
My opinion on that is I prefer straight cab because:
Bass appears to me more present (because highs and mids are not targetting your face) I use a VHT Deliverance 4x12 that already spread well enough despite being straight. Its front loaded and disperse quite good.
I prefer to have 1 single straight baffle focussing all the 4 speakers than a two sections angled baffle.
Yes a straight cab do look better but that should not be a factor...
If really slanted were necessary most 2x12 or combo amps would also be slanted when they are not; but some people do slant their combo on stage.
Another funny thing is that many tone purist (on forums) seems to favor the straight cab and use them but you will find 10 slant cabs for sale vs 1 straight in used gear sections. Is it reflecting really the sales? go figure...