Both UVB UV light therapy devices, 311nm and 308nm, have their own advantages in treating skin conditions, and which one is more suitable depends on the individual patient and the specific condition they need to treat.

311nm UVB Phototherapy is commonly referred to as Narrow Band Ultraviolet B (NB-UVB) therapy, which uses specific wavelengths of ultraviolet B light to treat a wide range of skin conditions such as psoriasis (psoriasis), atopic dermatitis (eczema), and other immune-mediated skin disorders. The 311nm UVB phototherapy device can be either full-body or topical handheld and is suitable for treating larger areas of skin.

308nm UVB phototherapy devices usually refer to excimer lasers or excimer light sources, which emit shorter wavelengths of ultraviolet light.308nm devices are commonly used to treat limited skin conditions such as vitiligo and psoriasis. Because the wavelength of 308nm is able to focus more on the affected area, it is ideal for treating small lesions. 

Overall, 308nm treatments may appear to be more effective in some cases, especially when treating hard-to-treat areas such as hands and feet, and because of its focused nature, it has less of an effect on surrounding healthy skin. However, 311nm NB-UVB therapy may be a better choice when treating larger areas of skin problems.