Both offer security and fulfillment, but as others have said, the focus is different.
If we are speaking about families in relation to the cards, I would say that the Ten of Pentacles represents a family that is comfortable (or more than comfortable) financially, and in most other matters. Things are good, but imo fairly stale. The 10 of Pentacles family is ruled by order and discipline...they sit down to dinner and keep everything clean even when there are no guests over...whereas the 10 of Cups family is bursting with love and companionship and generally (in my mind's eye) much less rigid. Their idea of comfort is to be together and have a good time, not to fit themselves into societal boxes of what comfort should be.
Does that make sense? Both families have reached fulfillment; the difference is what makes them fulfilled.
In a relationship, well, it depends on the position the card is drawn in. I wouldn't draw anything bad from either of these cards, but in the "present" position, the 10 of Pentacles to me would probably represent a mature couple who are thinking about and planning for the future...whereas I might see the 10 of Pentacles as more of an "enjoying the moment" card in a relationship. That's not to say there is no future. - Just that the distant future is not much discussed or thought about by the couple.
Both are positive, see? They just see different things in each other or the state of the relationship.