we race both Asso's and Losi's on our track in Holland. The difference between the two around a lap is very little. The Asso's biggest advantage is obviously 4WD giving it high cornering speeds and acceleration. The Losi seems to handle better on our track and seems to be easier to drive on carpet (with very high grip). The Asso's feels to hook (rear end overtaking the front end) which is very difficult to drive with.
The out-of the box motor from the Associated seems to be a little quicker than Losi's motor. You can make the Losi as quick by adding a Graupner Speed 300 at a price of only 10 USD. If you feel like buying a brushless anyway, you probably don't give a *** about that.
The Asso's need some more attention to the drivetrain than the Losi. We race under dusty conditions which enters the Asso's drivetrain. The Losi is a little more protected to that because of the closed tanny. I searched the internet for an undertray for the RC18T, and I found one at Ebay. For the price of 5 USD you can get a better protected RC18T, which I would like to advice you if you're not so much into doing maintenance...
These things made me decide to buy a Mini-T Pro. First of all, I think it's cool because it's a rescaled 1:10 XXX-T. The drivetrain is better protected and it handles better (at least on our track, doing no essential setup changes).
Nevertheless, the Associated is a lovely car as well and will be able to accelerate quicker and cornering speeds are way better (it's 4WD in the end...). Most negative thing about the Asso is that the drivetrain is open, allowing dirt to enter the gears easily. The Asso is a more popular car here in Holland, due to availability and the best value ready-to-run package.
Greetz and succes with your choice!