In most cases, double-sided means that the DRAM density is lower. For example, on a 128MB stick of SDRAM, a double sided is 64 bit DRAM, a single sided is 128 bit.
Since 64 bit (double sided) is more universally compatible it is, IMO, better memory.
Most 64MB chips are single sided, so it would be the difference between eight DRAM chips and four. They also make double-sided 64MB, but the same criteria would apply.
Modern 256MB sticks that are single stack are all 128 bit, and double sided. The older, double stack were 64 bit, and these should be used with earlier motherboards.
Russ, NCNE