Let's be clear: Posey is better than Daniels, so he would be an upgrade for us. If Posey gets waived, I'd love to make a run at him for the vet min.
Using some advanced stats:
win shares per 48 minutes (0.100 is approximately average): Posey 0.067, Daniels 0.064
Wins Above Replacement Player (depends partly on playing time, with Posey having a bit less than twice the minutes of Daniels): Posey -2.3, Daniels -1.3
Estimated winning percentage of a team comprised of the player and four average teammates): Posey 0.350, Daniels 0.349
Adjusted plus/minus: Posey 1.69, Daniels 1.80
For the sake of comparison (WS/48, WARP, Win%, Adj+/-):
Big Baby Davis : 0.084, 0,4, 0.441, -5.46
Tony Allen 0.100, 1.7, 0.507, -1.23
Then consider that Posey is more likely to decline, due to age, while Daniels is more likely to improve, due to the possibility he will be healthier than usual.