I agree with jennyrocks on this one. Both of these loves have their various limitations, but it is clear that the love between Joe and Pip is "truer", if you have to measure it that way.
It is clear from what Magwitch says about his reasons for "adopting" Pip that it has to do more with his desire to get back at a society that has excluded him rather than any real genuine affection or love for Pip. Though, at the end of Magwitch's life it is clear that they do have a very strong bond or love for each other, Magwitch is another example of a character in the novel who tries to influence and form another character for their own ends (such as Miss Havisham with Estella).
Joe, on the other hand, remains true to Pip throughout the novel. Although he is shown to be ineffective and rather a weak character at the beginning in his inability to protect Pip from the rampages of Mrs. Joe, throughout the rest of the novel he conducts himself with a dignity and grace that copes with the terrible way that Pip treats him, and in Pip's time of need it is Joe that nurses him better and pays of his debts. Joe's love for Pip remains constant in the novel, no matter how Pip treats or thinks of Joe.
It is my personal opinion that neither love is actually "truer" because they are both different types of love. But if you HAVE to pick one, I would personally go with Joe toward Pip. After having taught this a lot, I see the relationship between Joe and Pip as not only the father-son type, but as a friendship as well. Although at the end, Pip and Magwitch become very close, the bond was always there with Joe. Joe was a surrogate father to Pip, wanted to never embarrass him even though he did and couldn't help it.
When he was protecting Pip from his abusive sister, he would get the brunt of the attacks, and wanted nothing but the absolute best for Pip at all times. He even learned to write so he could write a goodbye note to Pip! When all of the characters around him were growing and changing, Joe was just...Joe. Dependable, reliable, Joe. He was the most faithful, true and honest character in the whole story and didn't GROW to love Pip, he just did from the get-go.