I'm not a fan of slimlines because to give any heft to the pens you end up with that muffin top look or wild shapes on the lower tube. That said the light brown pen looks best overall, I do like the grain pattern in the darker one though.
I make a couple of slimlines a year but I prefer cigar pens or majestic rollerball, I have made a few fountain pens but the kits were fairly cheap and I had problems getting ink to flow consistently through the nibs.