Dual-level generalized transition state theory and statistical calculations based on correlated electronic structure calculations with augmented correlated basis sets are used to predict rate constants and deuterium kinetic isotope effects for the competing S(N)2 and E2 reactions of ClO- with C2H5Cl. The E2 reaction is favored by zero point effects despite its higher barrier. Furthermore, the entropic contribution of low-frequency transition state modes promotes the E2 reaction by about an order of magnitude at 300 K, despite nearly equal combined electronic/zero point propensities for the two reactions. The kinetic isotope effect for the E2 reaction is predicted to be 8.3 at 100 K and 3.1 at room temperature. The kinetic isotope effect of the S(N)2 reaction is predicted to be surprisingly low at low temperatures (0.06 at 100 K) and 0.60 at room temperature. The activation energy for both reactions is predicted to be negative below about 900 K and positive above 1000 K. Variational effects on the location of the central dynamical bottleneck are small for both reactions.