I'm an artist by hobby, not for erotic art, but since you're probably looking at an anime-esque art style it should be the same idea. I personally use both sai and photoshop because they both make up for each others' weaknesses together. For instance, to me, nothing can beat sai at lineart, but I can't find a way to reproduce my preferred method of painting in sai, so I still like to do my coloring in photoshop. Imo if you can master both programs' strengths and weaknesses, and use them together, you'll be set for life. Just my two cents, op.