A rule of thumb I like to use is to multiply the bus speed by the Mhz obtained and take the one with the higher number. For example, a Pentium 150 Mhz running at 75 x 2 gives [ 150 x 75 = 11250 ] while a Pentium 166 Mhz running at 66 x 2.5 yields [ 166 x 66 = 10956 ].
The P150 will give better memory performance, but the P166 will do better for things that require raw CPU cycles (ie. crunching RC5 blocks).
Depending on use, I think the higher memory performance is generally preferred and if it were me, I would run at 133 mhz @ 1232.