Dao
No longer a newbie, moving up!
I have the 18-55 kit lens and a 75-300 zoom lens
Good, since your setup can cover focal length from 18mm to 300mm. I'd say try your existing lens and see which focal length (or range) you like the best.
Do not worry about subject isolation nor photo sharpest at this point. Just focus on what you want to shoot and what environment you think you will often shoot your portrait photos at. Indoor or outdoor etc. Pay attention to how the subject looks. For me, before I pick up my 85mm prime, I found myself often use my 70-300mm zoom lens instead of the 50mm prime because I like the way the photos look in outdoor. So I bought the 85mm lens. Now, if I have enough room, I often use the 85mm or the 70-200mm lens for taking portrait photos of my family.
But of course, there are other people prefer using a shorter focal length lens such as the 50mm. For me, some of the best family outdoor photos were shot at 200mm.
Once you find that you prefer a particular focal length or range, then look around and see what lenses are available that fit your budget. From that point, find one that best fit you.