It doesn't matter much which filter you get. If you get the "wrong" one, you will have gained invaluable XP that will contribute to your continued advancement in photography.

Options to consider:

• Don't get either filter. Since you will be using a tripod, you can use your current filter (or no filter) to take and blend multiple exposures. You can also play with other blending modes to decrease noise or increase resolution. (This is what I would do since I so infrequently dabble with long exposures.)

• Get the 10-stop filter, as scottbb recommends. Given his apparent experience with the topic, it would be reasonable to defer to his judgment.

• Get both 9-stop and 10-stop filters. You can delay your decision making until the moment before you take the photo.

• If there is a significant price difference, get the one that costs less. However, there is the saying, "Buy cheap, buy twice." (Perhaps this is why I have way more lenses than I can reasonably use.)

• Get a variable ND filter, as Tetsujin suggests. This would be the most versatile option, but now you have nearly infinite stops of light reduction to choose from. Is 9 seconds long enough? Or should you go with 10 seconds?

  My understanding is a properly constructed variable ND filter should limit cross-banding. But it seems like a potentially significant enough problem that it might be best to avoid them, as scottbb suggests.

• Get an ND filter set, perhaps a square filter kit, as Hueco suggests.

• Flip a coin. See TED: Barry Schwartz on the Paradox of Choice

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If you need longer exposures:

• Close the aperture a stop.
• Decrease ISO.
• Add your existing 4-stop ND filter.

If you need shorter exposures:

• Open up the aperture.
• Increase ISO.
• Switch from the 9/10-stop ND filter to your existing 4-stop ND filter.