Whoops, I read "I should" as me, not you Aella. I have no idea what you should do. I feel like my body composition is almost perfect but when I was younger I was little too scrawny and couldn't even gain weight when I tried to.
5 lbs of muscle wont even be visible noticeable, but you'll feel noticeably stronger. That's a win/win since your body is beautiful and I wouldnt change it.