What's the difference between 'Once Again' and 'Restart' modes?

[Xaks32]

What's the difference between 'Once Again' and 'Restart' modes once you finish the game and load up the save-file?

[talgaby]

Very small. Both give you all bonuses you unlocked in Leech hunter. I think Restart won't give you access to the machine gun and the rocket launcher, if you unlocked them through doing a speedrun.
The big difference is unseen: it resets the internal hidden statistics. Meaning that if you go for the all weapons achievements, Once Again will remember which handgun you modified with the special parts (Billy's or Rebecca's) and if you modify the other one, the achievement should unlock. With restart, that counter is reset. So is the counter for the read Files, so if you still didn't get the Fileophile achievement, you need to read all of them again, not just the ones you missed.