Should I stick to automatic or go to manual?

That's a good choice.
13's and 18's are just fancy 9 speeds... and nine speeds get the job done with one less shift than a 10 spd.
If you don't use the splitter on the side you have a 9spd, spit the high range and you have a 13, split both high and low and you have an 18.

 

Not much to learn.
Put the button on the shift lever down, and put in first gear, pull away, shift the common pattern, when out of gears, slip it into neutral, flip the switch up, as you ease it back to where 1st was originally, and repeat. The difference is while in high range, the side switch by your thumb now works to, so you can choose to use it, and split gears in half, or not, as you shift through the gears again in high range. There is no trickery, difficult concepts, or even 1 difficult thing involved. In fact it's so incredibly simple, its almost fool proof. Don't over think it, its as easy as falling off a trailer.

 

What's your application?

If you're running open deck, going to job sites and such, you don't want an automatic.

If you're pulling a van and all pavement, it's a matter of preference. If you know how to drive, you can get just as good or better fuel mileage with a manual as an automatic.

Also, remember there's more to figure than fuel economy. End up on the hook because some stupid fault code causes that automatic transmission to malfunction, and whatever $ you saved in fuel won't come close to paying that bill.