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Get the 55, it's way more common. The only real differences are the size and the availability and the 55 gallon beats it at both. Personally I think a regular 50 gallon is better than either though (it's basically a taller 40 breeder)
If the emphasis is on plants and if you want to get more bang out of the buck with your lighting, I would go with the 40L. On the other hand, more volume will provide more options fish wise.
OK, just to clarify, a 40L is not a 40 Brooder tank. My 40L is almost as long as 55 but 2/3 the wr idth. I find it good as a fry rearing tank but not much else. If plants are you thing go with a 40 Brooder. It will give you lots a floor space.
A standard 40 L is 16 7/8" tall and a 55 is 21" tall from aqueon. L and W are identical. As stated above if you are doing high tech plants I would do the 40L If not the 55 will be cheaper and give a little wiggle room with water changes.