Both of the coolers you mention have roughly the same volume, so the difference will be minor.
Have you considered a stacked-plate design with a built-in fan?
http://www.bmracing.com/products/category/?maincat=Coolers
That will provide much more cooling efficiency for the space, plus it will continue to cool with the same performance with the vehicle stopped. You can even mount it elsewhere if needed.
If over-cooling is a concern in your climate, you can add a cold-temperature bypass valve, such as the Derale 25011, which bypasses the cooler until it reaches 180F.