Nowadays double sided is so easy for manufacturers that it generally takes the same money as single sided. If you made at home, single sided is far easier to do,but you need to avoid issues with alignment, through-hole plating,etc. It also depends on how complex the circuit is - some things are very easy to route single sided, others aren't.

It depends on the soldering and shielding you need.

A double-sided PCB will have a ground plane which is better for reducing electrical noise.

I will keep on reading your sharing. Thats really interesting and helpful.