I wouldn't get too excited about this. Looking at their shot charts on Basketball Reference, they are virtually the same guy: very similar shooting stats both 2s and 3s, similar height and weight (Neal is 1" shorter and 20lbs heavier). The main difference I see is that Neal shoots from everywhere on the floor while Belineli shot from very specific spots both inside and outside the three which was probably a function of Chicago's offense.
One thing to note, this was an off year for Neal. He was struggling with foot problems all year. His production the year before is probably more indicative of his performance and in that year he was better than Belineli. Also, I really haven't seen much of Belinili's game, but I do know that Neal is a fearless shooter. If he's got the ball and he's open, he'll put it up from anywhere. That might be something we are going to miss.
Just from looking at the stats this is either a wash or a slight downgrade and since the Spurs are already over the cap, the salary issue is probably insignificant. Actually, this is probably a slightly worse option capwise because the Spurs wouldn't have to use an exception to re-sign Neal while bringing on Belineli will use up that space that could have been used on another player. Only time will tell though.