Stability constant is more for the complex:

Complex will be more stable if higher value of charge density is present.

$\text{Charge density = }\frac{\text{Charge}}{\text{Radius}}$

For stability:
(i) More basic ligand, then complex will be more stable.
(ii) Higher the oxidation state of central atom and higher is its coordination number.
(iii) Higher the stability constant (K)

$\begin{array}{ccc} & {\left[\mathrm{Co}{\left({\mathrm{NH}}_3\right)}_6\right]}^{3+}& {\left[\mathrm{Cu}{\left({\mathrm{NH}}_3\right)}_4\right]}^{2+}\\ \text{Stability}& \downarrow & \downarrow \\ \text{constant }\left(\mathrm{K}\right)& 5\times {10}^{33}& 4\times {10}^{11}\\ & {\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+}& {\left[\mathrm{Ag}{\left(\mathrm{CN}\right)}_2\right]}^{-1}\\ \text{Stability}& \downarrow & \downarrow \\ \text{constant (K)}& 1.6\times {10}^7& 1\times {10}^{22}\end{array}$