  Spin-fluctuation-mediated superconductivity is conventionally associated with
d_{x^2-y^2} pairing. We show that a generalized model of antiferromagnetic spin
fluctuations in three dimensions may also yield a state with formal ``s-wave''
(A_{1g}) symmetry but with line nodes at k_z \approx \pm \pi / 2c. We study
this new state within both BCS and Eliashberg theories using a realistic band
structure and find that it is more stable than the d_{x^2-y^2} (B_{1g}) state
over a wide range of parameters. Thus, models of spin-fluctuation-mediated
superconductivity must consider both possibilities on an equal footing.
