First off, for many circumstances, a 2 stop grad won't be enough, 3 stop is really the minimum and I often need to combine a 3 and a 2 stop grad. If you can only afford one filter, then a 3 stop is the one to go for first, but if shooting away from the sun, it may sometimes be too much. Whether or not you get a hard or soft grad (set) depends on what you are likely to be shooting. Hard grads are designed for scenes where the horizon (or other transition line) is straight, such as when the sun is setting over the ocean or when you have a single hill/mountain. Soft grads are designed for when the horizon isn't straight, such as when you have a mountain range, with variable heights.
As for brand, the Cokin mounts are adequate (plastic, but they do the job), but avoid their filters. I used to use Hi-Tech filters and they don't produce a noticeable cast, but they aren't in the same league as either Lee or Singh-Ray. I now use a Lee set, so my Cokin P sized reverse grad from Singh-Ray is no longer usable, but I used to use it all the time (sometmies when I shouldn't because it was so much better than my Hi-Tech filters), I never had any problems with colour cast. I keep looking at getting a larger replacment, as it is invaluable for sunsets, but the exchange rate isn't good at the moment, so they are expensive.
Here are some examples:
Singh-Ray 3 stop revers grad (with some HDR on the rockpool):
Fire and Water by
Kernuak, on Flickr
Lee 2+3 stop:
Kilve Pill under a Mackerel Sky by
Kernuak, on Flickr
