Jango is an exceptional fighter, and he is a quick shot. Assaj Ventress is no where near Mace Windu's level, and he disposed of Jango easily. But is this still enough to warrant him victory? No. Ventress has deflected shots from clone troopers, who have repeating blaster rifles. Jango has no chance in melee combat, and if he is stripped of his jet pack, he no longer possesses the aerial advantage, and thus, the range advantage. Not to mention Ventress is adept at closing ground quickly. It would be a 3-5 minute fight. But Ventress would win.