Depends partly on what lenses you are shooting, and what subject matter you are shooting:
If you are shooting general scenics with variable height horizons, and using regular to wide angle lenses, definitely go 2-stop soft gradiant. But... if you are shooting mainly flat-horizon images (ie: seascapes) and/or long telephoto lenses, consider the 2-stop hard.
On brands... assuming you are interested in quality of image: Would suggest Heliopan, Hi-Tech, Lee, or Singh-Ray. More expensive than others yes, but doesn't mess with colors and are made of optical quality materials.
On the Hoya polarizer you say you already have: If it isn't at least a Hoya HMC (Hoya Multi-Coat), and when your budget allows, you should replace it. The polarizing filter is THE most important filter in the stable... poor quality polarizers can be ruinuous to image quality. Poor quality polarizers causes color shifts, loss of contrast, blue-tinging of shadows, and will provide only limited range of polarity. I would suggest using Heliopan, Lee, Nikon, or Singh-Ray brand polarizers.