Assault marines are freaking awesome. They have slightly fewer upgrade options than Tacs, but they make up for it in several ways:
1) Assault marines can start taking special weapons at 5 men, not 10. This means that multiple small units of assault marines is a viable tactic. Small units of tac marines can't take special weapons at all.
2) Assault marines can take two of the same special weapon, making it much easier to create a "tank-hunting" squad or "infantry chewing" squad, versus tac marines, who must take different weapons.
3) All of an assault squad's upgrades are all Assault and Pistol-style weapons. This means that assault squads can always, well, assault. As opposed to tactical marines, who have Rapid Fire and Heavy options.
4) Jump packs give you unparalleled speed and mobility - and if you opt to remove the jump packs, you get a break on the price of their ride.