controversially i’ve never tried to watch them together, I dont think I would like it as I think the tone of each show is quite different, it is quite jarring when a character crops up in the other. I think a couple of the earlier ones worked, when there was still unresolved chemistry between buffy and angel but later ones just seemed shoehorned in.
Like they are both always facing apocalyptical situations, but always completely independently, you would think they would work together a bit more (know there is that throwaway comment about angel being the next line of defense or something), whenever there is an angel crossover into buffy I never get the sense of that character being the same one who has just gone through the events from the angel show, like how he turns up at the end of buffy and is still really into her, with all the stuff he had been through with cordellia and conor it seems weird, like this is angel of season 3 buffy not season 4 angel