This is tough...I don't think either one has it exactly right But, let's just say I find myself siding with the pebble more often than I'd like to admit.

I think the clod is parental love. Love them and let them go. Agape? I feel much the same about erotic love, in fact. I think it's a personality thing -- if I have to ask for something, I don't value it.
You ask such difficult good questions -- I'll be thinking about this one for a while.

Mary,
I was actually thinking along the same lines: that it depends on the type of love, that is, the object of the love (parent, child, partner...).
What do you think Blake intended? Are the pebble and the clod taking about romantic love (the way I took it in the end) or love in general?

I'm with the clod. The trick is to only fall in love with other clods. It's the pebbles of this world that will break your heart and do you in.

I can't help but think that if Blake were siding with the clod, he'd have put the the pebble first and the clod last; after all it's the clod that proposes the more acceptable 'romantic' version of love. What's striking about the poem is that selfless version is voiced first and the selfish version second, which means that it doesn't read as though the selfless version corrects the selfish version.
I suppose it's a cheat on Andrea's question to say that I think that love lies in trying to keep a dynamic tension between the two impulses. And that maybe that is what Blake intends.

"Without Contraries is no progression. Attraction and Repulsion, Reason and Energy, Love and Hate, are necessary to Human existence. From these contraries spring what the religious call Good & Evil. Good is the passive that obeys Reason. Evil is the active springing from Energy."
(Blake, from 'The Marriage of Heaven and Hell')

Not a cheat at all. This was a great comment. And I love the quote.

Both sicken me, and I would probably feel more contempt for the clod as long as the pebble is cruelly manipulative rather than needy and whiny. It's easier for me to look at trampling boots than at a willing doormat.

The clod and pebble describe a tug-of-war, with one side passive and the loser picking up the slack. The mounting resentment and passive-aggressiveness will create enough tension so as to eliminate the slack and create a stalemate.

What is between a clod and pebble? Love should mean you begin and have as/at base an equal footing with your beloved; you're kind and loyal to each other. You want for the other person what (s)he wants for him- or herself. You're on each other's side, so while you may disagree, you're on the same side. There is give-and-take, but it's not a tug-of-war: you're both pulling in the same direction.

I always liked Iris Murdoch's saying that "Love is the difficult realization that something other than oneself is real."

So I would have to say the egocentric version of the pebble. If the compassion from acknowledging that someone else is real leads to kindness, it is a fortunate consequence.