I would personally go for a true long board. A Phantom, a Rio or a Kona One will have better glide in low wind. My girl (11), prefers it over a short board (Starsurfer) we have because she enjoys the faster glide. I'm now sure that I will always have a longboard available for days with variable or low winds. (Most of the time here...)
The 171 liters will be planing quickly, but may not perform well as a longboard in low wind because of it's width that will cause extra drag. You will want to replace it in a year or two because the daggerboard will be more of a safety than a performance feature.
If you don't have storage room for a longboard, then go with the 151. Roger explained well how you can use it, and it will be closer to a low wind shortboard that you will want to keep in the future. It may be cumbersome at the beginning to add and remove the center fin, but since it's use will eventually stop, you will have a nicer low wind shortboard in the future.