I haven't geoprocessed that many features since v9, but right away would lean toward using raster. You might be looking at overnight runs to let the machine churn through the large number of records.
A factor to consider is how precise your data must be. Consider the number of raster cells you will need versus the number of polygons you'll process. If you need 10 million grid cells, but only half a million polygons, the latter might work better. Are your slope values from a 30m DEM? Your grid cells should not be less than 10m. If from LIDAR with cm accuracy, this would give you vastly different needs.
Raster processing would be simple, really. Assign values 1-6 for Input A. Add this (using the raster calculator) to values for Input B (values in multiples of 10) so that the possible output values are unique. Every combination of A and B has a unique sum in the output.