Re: 5mm or M5/6 for Arches??
I've seen arches with both styles and think both look great, but I like the M5's better, they are viewable from any angle. It's true, the 5mm are very directional, but when viewed dead on they are very bright (much more than an M5). It's a trade-off.
I faced the same decision when making my mini trees this year and chose the M5's. Very happy with them.
X3 Ren48LSD, X4 Ren16SS, X2 Ren24SS, X1 SRen32, FM-02, E1.31 Bridge
X36 Super Strips, X9 Dumb RGB Mini Trees, ~25K lights (mostly LED)
Pixel Mega Tree with 1600 pixels (4800 channels) run with an E682