Pinnacle has released a "transition guide" that has the bare bones rules that have changed between the first and second edition, along with a couple other rules that are common to both to make it a bit easier to follow. The idea is that with this document, you can continue to use the first edition with all the updates to the second edition. This means that you can get the rules updates and you don't have to buy the second edition.
Note that these are just the raw rules and it doesn't include "fluff". For instance, power tiers are listed, but not what sort of heroes might exist in each tier. Similarly, it misses a few descriptions of character creation steps (e.g. skills, how much money you get for gear) and doesn't provide descriptions on technology levels for gear.
Several reasons you might want to consider buying the second edition include:
- You want all of the information in one place, rather than between the first edition and the transition document
- You want something more visually attractive than the bare bones transition PDF
- You want to include all of the "fluff" that has been cut from the transition document
- You want example NPCs and Headquarters that are updated to the new rules
So there are reasons to buy the second edition and reasons to stick with the first plus the transition document. You'll have to decide for yourself whether it's worth it for you.