Essentially. Core Set is all of the old runes, making them typically bad. Extended set is all of the more recent runes, making them typically good.
To give you a real explanation, core is sets 1-6, extended is 7 through the one just before the current set. You can see this by clicking the question mark on the top right of the item in the marketplace.
Can get angels from core. The question mark near the top corner of the boxes when your about to buy them actually tells you what sets they offer. It's so hidden though it seems a bit misleading. If you want new runes and potentially more trade power get an extended if you want old runes and a much slimmer legend pool that can potentially give big returns get core. Heck the core box I grabbed recently offered almost double the value from the 2 legends I pulled vs the 2 legends I pulled from extended.
The Rune Checklist says the diplos were the only legendary champs released in the first six expansions.
I got Draksar conslate, Angel of Protection, Cyclonic Fesh and Aspect of Growth. It looks like I ran to many expansions together when I opened all 20 packs