I like the grab straps that attach to your saddle, but – and this is a big one – if your saddle slips and you need to grab something, you’re SOL.
So I like neck straps better. I agree the best one is a retired stirrup leather. Punch holes to fit, and thread the extra length through the metal of the stirrup buckle.
Depending on your anatomy, a hunt plate or polo breast plate might have better reach.
C4 has fun belts in a bunch of patterns that are used as neck straps, a ton of fun colors, seem very popular with some but I am a traditionalist.
You can also use old or tired buckle reins for a thinner, nicer look. This is a bit more involved than using a stirrup leather, as you have to do some measuring, hole punching, and a little cutting.
P.S you can use a recycled tail strap or old flash/spur strap to keep neck strap affixed to your martingale or saddle. I add a snap (pictured below) to the flash/spur straps to make it easy to clip on/off the saddle’s D-ring.