18 speed vs 15 speed

If the truck is what you want then go for it.
I got a 15 in one of my gliders behind a 550 E model with 600/2050 file in the ecm.
I wouldn't be worried about the trans not taking the abuse , there perty stout.
Some times it would be nice to split the top gears but overall it works well.

If your still not happy a trans swap to a 18 is no big deal really.

 

My first truck had a 15 double overdrive in it & it was bullet proof pulling a hopper bottom @ 100,000lbs. My last truck had a 13 & my current truck has an 18. Maybe its because it was the trans. I learned to drive on but I'm still partial to a 15, I like the deep reduction too for pulling out of soft fields.

 

My Fathers Old Freightliner Flc had a 15 speed behind a B model Cat.. You will love that Tranny if you ever have to go off Road.

 

I just got done pulling OSOW daily up 155k, thru sand, dirt, mud,like this-

and (occasionally) hardtop with a 15, and like Granhawler says, that deep reduction is nice to get out of the nasty stuff. I just wish when my boss added the 4th axle for HH, he would have upgraded the drive axles as well. The tranny has held up fine for over 2 years now; however we're on our third one of these....

 

15 speed for offroad,18 for on road as you can split the gears.

 

No bid deal but you didn't say if its a 15OD or 15 direct. I'm not sure what the advantage of the deep reduction is....IIRC there isn't much if any difference in the ratios from the 15 and 18....1st gear might be a tich lower in the 15. Personally, I HATE the 300+ rpm steps between the gears on the 15OD. 15 is probably a more durable trans because of those syncros in the 18.

Last year, Domar in Toronto quoted me $5,000 + my 15OD as a core. Then you'll need to add your mechanic's labour.