Having read both of these authors, I can confidently say that Bell and Tullian are significantly different in their approaches to scripture, and in their interpretations thereof. I align myself strongly with Tullian's approach, but disagree strongly with Bell's belief.
Historically speaking, Bell's theology aligns well with historical Universalism, or Universal Reconciliation. He would state that all people, regardless of belief, will ultimately be going to heaven. To accomplish this, he redefines sin's effects as being bad not because it is an offense against an infinitely good God, but because of what it does to us. Sin damages us, but does not condemn us. And since it does not condemn us, the cross of Christ cannot solve a condemnation problem, but instead becomes a good example by which we live our lives.
Tullian's position falls pretty clearly along the lines of Calvinist Protestant Theology. He believes that the law of God is clear and extensive, and that the consequence of disobeying the law of God is eternal condemnation. Likewise, he believes that Christ's death on the cross fully paid the penalty that we owed for our sin, so that, through Christ, we might be able to experience fellowship with God. He believes that Jesus death on the cross grants positional righteousness to all who believe, thus justifying them in the eyes of God, and removing from them all condemnation.
Bell focuses on grace in the absence of the law, and in the absence of prior condemnation, but Tullian focuses on grace in the presence of the law, and in the presence of prior condemnation. The problem with Bell's theology is that grace cannot truly be real if the problem of condemnation doesn't exist. It would be as though you threw a life preserver to someone walking down the street in order to save them from drowning. Being on the street, they were never in any real danger of drowning, so they have no need for a life preserver. Tullian's theology, in contrast, offers a real problem, and then offers a real solution to the problem. We are condemned to hell, but Christ came and paid the penalty of condemnation, so that we might be free from that penalty. It's like throwing a life preserver to someone who is drowning. It makes Christianity needed and useful.
In short, Bell takes a real, deadly problem, denies it exists, and thus makes Christ nothing more than a symbolic, but unnecessary, solution. Tullian does not.
As a side note, it's important to that Tullian is mainly concerned with justification's effect on sanctification, while Bell is mainly concerned with defining justification, so objections to each of them are pointed at these two approaches. Piper's objection to Bell is objecting to Bell's view of justification, while Murray's objection to Tullian is objecting to his view of sanctification in light of justification. So in this sense, Piper objects to Bell saying that Bell is not defining Christianity right, while Murray objects to Tullian saying that Tullian doesn't define how we are to live out the Christian life right, yet he would agree with Tullian on his view of justification, and even says so in his objection.