Factory team Gt all the way and I will try and explain without trying to start any flame wars.
Mugen Mst-1, while a very capable truck the thing is just plain week everywhere and every pivot point WILL be worn out in a matter of a couple of tanks of fuel. I owned one and raced one for about a month before I sold it on ebay cheap. Its kinda funny how if you look on ebay you always see mst-1's on their with the heading Mst-1 very little used for sale in good shape, thats becasue it doesn't take the average racer very long to figure out what a pile it is.
XXX-nt, another capable truck, Not much to really coment on other than sometimes the tranny can be a pain (alum. diff gear not the answer because it actually messes up the tranny worse) and the plastic/graphite can be kinda hit or miss, sometime they will take the beattin of the week and other times you look at them funny and they break. The losi typically handles rougher stuff better than the "stock" gt but the rear of the truck lacks traction quite often so sometimes it can be a handfull
Factory team gt, well what can you say other than this truck has won every title out their and is the 2 time in a row reigning Roar national champion. While the truck is dated why mess with a good thing. The Gt is by far the toughest truck to be had and has the most rear traction ( makes 2wd easy to drive) You do need to add a receiver box but thats about it, it just plain works well. With some simple tuning it DOES turn just as good as a losi but you will most likely always finish your race.
Engines. I always recomend the O.S. red head cv for a first time truck guy it will make driving much more enjoyable while getting used to the trucks handling especially on a dry day. I also really like the mugen mt-12 with a O.S. rotary carb. As for pipes the Adam Drake pipe is very good with the mt-12 and the factory team gt pipe is very good with the cv. Just remember their is no reason to spend tons of cash on a engine for a truck you simply can only put so much power to the ground and to much power will be a handfull.