I have both, CK 2 is better as far as I'm concerned. They are both very similar, CK 2 is an iteration on CK and pretty much better from what I've played of both, only bad thing is the absolute wealth of DLC available for CK 2 which would make the whole game very expensive if you wanted it all.
I only got the base game and will look into the DLC if I love it. Thank you Adalia
I have the base game (it was gifted to me by a generous GOGer) and i haven't been bored even after 30-40 hours (edit: now that i think about it, it must be way more than that :P) that i've already played. You definitely don't need the expansions as the game is perfect even without them; only buy them if you're interested in a specific faction or if you get bored and want to try something new. And you most definitely don't need the cosmetic DLCs, unless you pocket is deep... Personally, i'm very interested in The Old Gods expansion since you can play as a Pagan with it plus you start at an earlier age, but unless there is a GOG version of CKII, i don't see me buying it.
Having said that, CK is on sale several times a year and at a great discount too, so you may want to try it even if it is only to compare them yourself. You don't need it to enjoy CKII.
TIP: don't forget to check out the unofficial manual: https://forum.paradoxplaza.com/forum/index.php?threads/my-crusader-kings-ii-manual.693266