Leather looks great but it's warm. It's also a pain in the butt to sew. If you don't have a sewing machine built for leather work you'll need to hand-sew. It's doable but it's tedious -- but what part of making these outfits aren't? Leather's also expensive. Pleather is cheaper but it's pleather.
UnderArmor is good but if you're skinny like me it might look odd unless you bulk it up somehow -- either yourself or the suit. Some people wear those "Green Man" suits in black. Similar idea.
I'm planning on doing a leather-mesh mix... leather where there would be the extra padding (shoulders/knees/elbows/abs/etc.) and mesh at the inside of joints and other spots so it breathes a bit more and can bend easier. I'll probably wear UnderArmor underneath all that, too, but we'll see. These suits can get warm so anything to help with airflow and keeping cool is A+ in my book.
There are a lot of good ideas floating around on the forum. Mix and match and see what works.