Ace Mode is just essentially "hard" mode for Space Ace. But it's not very different. You can still give yourself five lives, and you can still continue on game overs. The only differences I could pick out while I was doing it are:
- You have less time for the prompts, it's a lot faster than normal difficulties
- Some scenes seem to repeat themselves after, essentially making some parts double the length they usually are
Other than that, it's the same thing as playing on the other difficulties, just pay attention to the prompts and push through it