From any angle, they look the same. Both have the same shape, same suspension system, and same accessories. However, the TBH-II is an aramid ballistic solution and the FAST relies on a Polyethylene solution. The TBH-II offers 9mm protection at 1400 fps and the FAST offers 9mm protection at 1200 fps, so fairly similar balllistics. The real differences are in weight and cost. The TBH-II weighs in at about 2.5 lbs while the FAST helmet is 1.5 lbs, both as configured. Now is the biggest factor for many agencies; the TBH-II is $599 and the FAST is in the $1400-1500 range.
If you’re looking for mission configurability, and the cool guy factor, but are willing to accept higher weight, then you may want to consider the Gentex TBH-II MC.