Well, these devices both measure light intensity:
OPT101: http://www.ti.com/lit/ds/symlink/opt101.pdf
TSL237: TSL237 Light-to-Frequency –Light-to-Frequency Converter | ams
The difference lies mainly in how they present the light intensity at the output.
The OPT 101 delivers a voltage which increases linearly with intensity.
The TSL237 delivers a frequency (Square wave 50% duty cycle) proportional to the light intensity.
You can work with both using an arduino. The OPT 101 you can read maybe with a voltage divider direct by an analog pin. The TSL237 is a bit more complicated because you have to write code to measure the signal frequency (up to 1Mz which could be around the limit without simple additional hardware such as a frequency divider chip)