I have both Voyager and GT series...on an avg voyager writing speeds max out at arnd 6MB/s whereas its arnd 9-10MB/s for GT series. The speeds achieved with voyager and GT are much higher than what you'd get from transcend/sandisk/kingston ones. Read speeds are way higher than such run of the mill pen drives.
The speed on GT is more than 50% less than what you'd get from even the slowest portable drive.