Either one. If you get grids with them get the 40cm strip softboxes otherwise which to get depends on things like how much space you have in your studio.
The primary use for strip softboxes this size are for rim lights. The narrower one can make getting narrow rim lights easier while the wider ones will give you a wider rim light if used at the same position.
Do you have the distance to back off the 40cm strip softbox far enough to give you a rim light as narrow as from the 25cm one? Do they come with grids?
Position the light behind the subject and adjust how far to the sides until they are out of the frame but give you the amount of rim lighting you want. You have some control over the width of the rim light by how you position the light.
These will be very awkward to use. They may not exceed the weight capacity of your light stands but when you place something that long on a stand you get a lot of torque causing instability.
I have two 14"x56" (35cmx140cm) strip softboxes and the only way I can use them safely is to mount them vertically with the lower edge of the softbox resting on the floor.
If you want to use one as a hair/rim light you can consider mounting it overhead but to me these would be too long for that unless you are shooting groups.
There is one other way to use two strip softboxes this size that I have tried. Place them on each side of the subject and aimed at each other. By moving the subject to the left or right you change which is the main light and which is the fill light and the lighting ratio. By moving the subject forward or backward and/or using or not using grids you change the character of the light on the subject. Lots of possibilities but you need to experiment to figure out what works and what doesn't.
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Living and loving it in Pattaya, Thailand. Canon 7D - See the gear list for the rest.
