You haven't given enough information to produce a definitive answer, but here's how I suggest you can help determine the information yourself. I own and use 2 sets of ND grads from 1–3 stops, both hard-transition and soft-transition, plus a few specialty transition ND grads (reverse ND grad,
"sunset strip" ND, 0-transition/continuous ND grad).
Hard vs. Soft Transition
Hard vs. Soft comes down to focal length of lens you want to use. Soft transition ND grads are usually used with wider angle (i.e., short focal length) lenses, whereas hard transition ND grads are used with telephoto / narrow FoV (field of view) lenses.
Of course, there are exceptions to the rule, whether they be for certain technical reasons for a particular scene or for artistic reasons. But simply, the wider the lens you use, the more hard the ND grad transition looks. If you are shooting a mountainous horizon where the transition line isn't straight and obvious, you probably want to go softer rather than hard on the transition. But if you are shooting an uneven horizon with a long lens (say, 200mm or more), a soft ND grad is probably too soft — you can't actually pick out a more-or-less transition region in the ND grad; it just becomes a 1– or 2– stop transition from one edge of view to the other, rather than a region of constant 1– to 3– stop reduction transitioning into a 0-stop region.
Number of Stops
To determine what you need, even before you buy ND grads, you should do the exact same thing that owners of ND grads do when they decide which ND grad to use: meter the scene to determine different exposure levels.
Assuming you are just using the camera's built-in metering:
- Begin by metering the foreground exposure.
- Then meter the sky (or whatever the region is visually different that you want to cover with your ND grad). Don't meter the sun if it's in the scene. Just meter a representative portion of the sky.
- Select the ND grad that reduces the difference between foreground and sky exposure to within 1 stop or so.
Without even buying a filter, you can go out and meter a typical scene you'd like to shoot. That will give you an idea of how stops your ND filter should be.
I live on the Florida seashore (east coast), so I have ample opportunities to shoot sunrise pictures over the ocean. I am also surrounded by an intercoastal waterway, so I can shoot some okay sunset pictures over water as well. Generally, I find that I use my 2-stop ND grads more than my 1– and 3–stop ND grads.
However, if I am shooting sunrise or sunset with the sun in the picture, I reach for either my 3–stop ND grad, or my "sunset" ND strip filter (or a combination of strip and 1– or 2– stop ND grad).
If you are primarily going to shoot with a normal- to wide-angle lens, I suggest getting a kit of 1–, 2–, and 3–stop ND grad soft rectangular filters. If you are going to shoot with a mix of wide and long lenses, I suggest getting both a 2–stop soft and 2–stop hard ND rectangular grads.
Why rectangular grads? Because it allows you to frame your camera as you wish, and then set the transition line where you want it. Circular grads have the transition line in the middle; therefore, your composition is dictated by where you want to place the transition line.
Additionally, rectangular grads allow you to combine them in interesting ways. Wish you had a ND strip filter to kill the bright sun near the horizon, but you only have a couple 1–stop ND grads? No problem:
If they were both 2-stop filters, you'd have a ND2 strip + ND2 solid filter, giving you a 2-stop reduction of the horizon relative to the rest of the scene.
This is just one example of how you can creatively combine ND grad filters.