I've been monkeying around with trucks lately too. I first did a Monster GT and now I'm playing round with a Losi Muggy. Both go at around 13lbs, so they are heavy with the 8Ah batteries that I run in them. I can say unreservedly, go with an 8XL or similar motor. If you get a Neu, give some serious consideration to either the 2000kv or 1900 kv (I think it's supposed to be more efficient) motor, or maybe even the 1700 if you go with the 1515. You do not want high rpms in a truck and I think that anything over 2000kv in a truck is high rpms.

If you want to be able to throw the largest brick of a battery into it and run until your fingers are tired without worrying about overheating anything, just keep it solid and don't go overboard. I started out with a 7XL in all five of the vehicles that I did and have since gone with the 8XL and that class of motor and am happier for having done it. On the plus side, you can gear the lower kv motors more and if you ever get the wild hair to try 5S, a sub 2000 kv motor would be the ticket for a rocketship ride. If speed isn't a primary concern, there's nothing wrong with a 9XL either. That and a MM controller would be cheap and whole bunch of fun.