i'm only familiar with the mugen and the xray...
the mugen is a great truck... the prospec is loaded with aluminium parts... handles well and last a long time. it's a pivot ball design, which is what i dun like... weights heavier than the xray...
the xray is very light, very nimble in the handling... last a long time too.. the reason they do not have that much aluminium parts is to keep the weight down... anyway, they dun need them... drive train on the xray is bullet proof...
considering that you are in the usa... you should think about parts support... i think in the usa, the mugen is a more popular car... thus easier to get parts either online, or in the hobby shops... go with the one that has support in your country, or shall i say, more accessible support for you...
2 cents...