Even though the Arcade Mode and the Campaign Mode is almost the same, there's bound to be things that might be vital to finish the game and some pointers that will help you finish the game especially when it comes to using weapons and the difference between Wii Remote and Nunchuk and the Wii Zapper.
The main difference is with the Campaign Mode, you move around at your own pace. Also, the levels leaves you more things to do but has hidden objectives that has yet uncovered from Arcade Mode.
The Arcade Mode is more smoother than the Campaign Mode. What's not included that would ruin the pace is the ability to tune the radio and go through the minefield. Also if you want to crouch or duck behind cover, you have to use 
to do so and again to stand and shoot.