My wood mallet is the inside scrap from cutting a heart shaped box.
It's a heart shaped piece of walnut about 2" thick and maybe 5" wide at
the "hips". For a scribe/marker, will that 2/1000' difference between
a scribe and a sharp pencil make a difference?

With measuring tools and straightedges, it can make a huge difference
in your project, but will owning a better wooden mallet make you a
better woodworker?

I'm as guilty as anybody for getting hung up on marketing hype. I have
13 guitars to remind me that a different neck, pickups, or fretboard is
no substitute for practice.

However, if what you do requires 1/1000 precision in your scribe lines,
I'd recommend Steve Knight at www.knight-toolworks.com. His scribes
are custom made to order, so you can specify exactly how you want the
blade angled and which side it is sharpened on to fit your style.
Steve is an excellent guy and makes excellent tools, so I hate to say
anything derogatory, but I have one of his scribes and honestly don't
use it that much. A sharp pencil gives me enough accuracy and is a
heck of a lot easier to see than a scribe line.

As for wasting your money on a wooden mallet...build your own (I can
send you some plans for a heart shaped chunk of wood *g*) and put the
money you saved towards the best tri-square you can get. I like
Starrett, but other people may have different opinions. Gather the
information available and see what works best for you.