depends on how many Watt/USG you want to achieve.
For high light planted tank (with riccia etc), 3W/USG (or more?) is what most practises. In this case, PL begin more compact, will be a good choice. I have 4 x 36W PL for my 3ft.
For lower light planted tank, you can either have FL or fewer PL to ahieve 2 - 2.5W/USG.
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