I only carry one grad grey but am considering getting two, I usually use the P121 but have found that the increase angle of view from ultra wide lenses mean more of the very dark part of the filter appears in the frame at the top. The P120 is a bit more subtle. For 28mm and up the 121 will give both subtle and heavy effect.

I leave the cokin holder on my 12-24 lens all the time and have ring adapters fitted to all the other lenses so swapping the filter is easy. The ring adapters are very cheap. I just keep the filter in its box in my pocket and take it out when needed, takes all of about four seconds.

When you go the opposite end, using telephoto lenses, the grad filter has almost no effect since you are using only a small part of the surface of the filter. If you need to use them with a tele lens you will need a hard stop ND grad which are harder to come by.

Lee filters are much better quality and a little more resistant to scratching but horrendously expensive.