The Q40 has a reputation as a basshead headphone but their mid and treble frequencies are not recessed, they are actually quite neutral and monitor-like. Basically, they are a neutral headphone with big bass, but the bass is quite high quality, considering their price.
I'd say that the PRODJ100 is probably slightly more resolving and also not quite as capable in the bass region.
They are both good headphones for the price and I am sure you'd be happy with either one of them; however, do keep in mind that replacing the pads on the Q40 is basically a necessity (as the stock pads are not very good) and this means more expense.