This is a great question! As you can imagine, there will be difference of opinion. However, I think the point of the story is that no one made the bigger sacrifice. In other words, we are not supposed to think of the sacrifice so much from a commercial point of view but the love. Let me explain.
First, Della and Jim both gave their most prized possessions. This alone should tell you that there is parity in their sacrifice. If a person gives their best, then that is all that they can do. To use a different metric would do harm to the giving. In a word, it would vitiate purity of the giving.
Second, the whole point of the story is to show that when two people give, then all stand to gain. Della gained in that she expressed her love in a great way. Jim gained as well, because he, too, showed his love. To be wise is to seek your joy in the joy of others.
While the general message of the story is that both Della and Jim sacrificed their most prized possessions for the person each loved the most, one can make the stretch that one character sacrificed in greater measure than the other.
Jim's sacrifice appears to be the greater one for the following reasons.
1) Della sacrificed her hair. While it may take several years for her hair to grow back to the length that it was when it was cut, it will indeed grow back and before long she will be using the combs that Jim bought for her. Thus, Della's sacrifice was only temporary.
2) Jim gave up his watch to purchase the combs for Della. Unlike Della's hair which will regenerate itself, Jim's watch cannot return in time. As poor as the young lovers are, it is unlikely that they would be able to generate the money necessary to go back and purchase the actual watch that he sold any time soon. Odds are the watch will be sold to someone else in the meantime. So in order for Jim to ever use the watch fob purchased by Della, he will have to earn enough money to purchase a different watch and it will not have the sentimental value of the watch he sold.
Thus-- Jim is the character that sacrificed the greatest.