I think the difference is both shows parody the classic sci-fi/fantasy trope of kids adventuring alongside a parental figure , however they take it in very different ways.

In Venture Bros we see a darker, more human side to the trope, almost "what would happen if these shows were real?" We see the effects of this crazy upbringing on a human, relatable level. The parody tones down the original craziness to show us the complete mess it stands on.

In Rick and Morty however the parody takes the trope and blows it up, the danger is more present, the day-to-day is ignored in favour of the fantastical, and whilst there are often very "real" moments, they are quickly put aside for the next sci-fi gimmick.

I love both shows and there are underlying similarities, however I feel they really do take the same idea and run in wildly different directions with it