Weight of 13sp vs 10spd?

Depends on whether you want to save weight, or have more versatility.

A 10 speed weighs less than a 13 speed. A 13 speed is better able to keep you in a good power band.

I have a 15 speed in one of my trucks and a 10 speed in another.

With the 15 speed, I rarely split the lower gears, and in fact, often skip gears on the low side. Sometimes I skip a gear or two on the high side, as well. In the areas I run, I usually drop out of overdrive and pull the hills in direct gear.

With the 10 speed, which is a Lightning T2, I usually opt to start in 3rd gear and never skip a gear on the way up. Similar to the 15 speed, when pulling hills, I drop out of OD and run in direct gear.

I guess it kinda depends on what weights you're running, where you run and the rest of the drivetrain. What works for a fleet truck hauling bread from the bakery out to distribution centers doesn't necessarily work well for the truck hauling 80k gross on the same route.

 

...now if you had a Mack 13 speed, all you'd have to do to convert it to an 18 speed is remove the pin blocking the use of the splitter in the low range from the shifter knob. That pin in the knob is the only difference between the two.

Eaton transmissions are different, though...so if that's what you've got, you're stuck with it.