[Grammar] "Both of them" or "Both of which"?

[ngoc_lan]

Can you tell me how to use "Both of them" and "Both of which"?
Which is the best choice for this sentence?
Although solar energy is cheaper than oil, _________ have advantages as well as disadvantages.

If you don't mind, please explain in detail and give me some examples so that I can understand clearly.
Thanks for all your help!

 

[SoothingDave]

"Both of them" or simply "both."

"Each" may even be better. "Each has..." Because they don't, together, share the same advantages and disadvantages.

 

[ngoc_lan]

Thanks a lot