I have both the Ultra Plush and Ultra Smooth. The Ultra Plush has a noticably higher pile than the Ultra Smooth. Other than the colors, that's the only difference that I have noticed.
I've used the Ultra Plush for applying and buffing spray sealants and it seems to work well using the plush side to apply and the finer side for final wipe. I had purchased the Ultra Smooth with the intention of using it for buffing, but I am currently using another type of MF towel for buffing, so I'm not using the Ultra Smooth for anything right now. I might try it for applying spray sealants as it is very plush but with lower pile than the Ultra Plush and that might absorb less spray when spreading it around.
Overall both are very nice.
I don't have the Ultra Soft WW, but I understand that it's softer and slightly thinner than the regular WW Glass/Drying.