[MENTION=25394]sonictard[/MENTION] 7 and 8 are very different in my opinion, that may be because I have read deeply into Enneagram ( which I believe is the best, and only way to truly learn the system).
First: Tests are dumb, unreliable and should only serve as a jumping off point--
far too many people take the test, get a result, smack them selves with a label, and call it a day without ever really understanding what that means. That's abusing the tool out of negligence and leads only to stereotyping and misunderstanding.
Now back to your question. If you feel like you may be 7 or 8, then you've probably found core + wing. The key to determining which is which, is in deciding which motivations are more dominant in yourself.
7 is more bouncy and energetic, they tend to have issues with focus, and crave general variety in their lives. The reason for this (
motivation), is to distract themselves from any sort of pain-- specifically serious, dark, or painful thoughts, conversations or interactions with others. They just want to have fun, see the world and focus on the positive. The core sin of 7 is Gluttony, or overdoing/indulging in basically everything.
8 is more grounded and solid in presence and tends to come across as a dominant, independent and leadership oriented person. 8's can also have the reputation of being jerks, as the often challenge and push the buttons of others-- the reason for this (
motivation) is because Challengers believe they must dominate and gain control to survive. The core sin of 8 is Lust, and usually manifests in their desire to control and overpower everything in their environment-- destruction is often a part of this.
These are super brief descriptions. As I said, you should probably read up more on the types,
The Wisdom of The Enneagram is a great starting line.
Since you mentioned 3 as well, here's a post I wrote on the differences between
3 and 8, and
3 and 7.
As to your question about aggression, any number of Enneagram types can be, just motivated by different things.
Hope that helps!