I drive a sigma4 on a regular basis. It's not underpowered at all. Infact, the low range torque is quite fun. It's 0-100 time is around 10s, which is not fast by any standards but is adequate. The updated engine is confirmed coming in both hilux and Fortuner facelifts. That should bring the 0-100 time to 9.6 to 9.8 secs.
Toyota has pulled the plug on 4.0 v6 Fortuner and Prado in not just Australia but in Japan and Europe as well due to the stricter emission control. They still offer it in the middle east since the Arabs don't have such strict emission standards.
Saw an Australian review of a Sorento 3.3 v6 and its 0-100 numbers were 8.5 secs. Ours is not the same engine but has similar power and torque figures so the acceleration time would probably be similar. Sorento is faster because of the lower kerb weight and higher bhp, Fortuner would have a stronger pull under load and initial acceleration though due to the higher low-end torque usually typical of diesels.
For a city and highway driver - Sorento will always be a more comfortable choice. Fortuner is not a direct competitor, its a properly equipped offroader. A camry-based Highlander/Kluger is better suited for this purpose.