Its not worth buying both now imo, they are very similar.
The important things you will miss if you go for #1 only is domestic policies, cultural zones, ability to choose options at historical events and a few diplomatic options, like guarante a nations independence, create vassals and a few more. On the other hand #1 seems to be balanced to follow history better then #2.
The domestic policies provides a few interesting options, and gice quite a large effect on how you play your game if you use them right, but they arent that much fun imo.
Cultural zones makes it more difficult to expand since owning provinces of "wrong" culture makes them less productive.
The ability to choose different things when you get an event is very fun imo. In some cases there is one option that is clearly better than all others, but that doesnt really ruin anything.
Some of the diplomatic options are a relief, but not all are that usefull.
Personaly I would go for #2, but #1 is definatly an enjoyable game.