SIM868 is a more advanced, cheaper and smaller version with improved specs compared to SIM808 and below are the differences i observed between them,
SIM868 has DUAL SIM card interfaces :
SIM808 and SIM868 are both quad band 2G modules and can work globally in 850/900/1800/1900 MHz frequency bands without any problems.
What sets SIM868 apart is that, it is having a dual SIM card interface, which means, you have the possibility of connecting to two networks when the other is not available or you just chose to.
SIM868 is super small:
SIM868 is notoriously small in size and comes in a form factor of 17.6*15.7*2.3mm dimensions where as,
SIM808 follows the footsteps of SIM900 module series and comes in the form factor of 24*24*2.6mm dimensions.
SIM868 has GNSS by default & not all SIM808 have GNSS :
The SIM868 module has GNSS functionality where as SIM808 has GPS functionality with GNSS support being present only for hardware release version V2.01 and above and software release version 1418B01SIM808M32 and above. So make sure your module meets this requirement to use GNSS features.
SIM868 has better GPS tracking sensitivity :
SIM868 module has -166dBm tracking sensitivity compared to -165dBM of SIM808 module which helps it in locking to GPS signals even in weak signal environments.
AT Commands compatibility:
Since both the modules are of SIM800 series, they are AT command compatible, in fact there is a single AT command manual for both modules.
SIM868 is cheaper :
SIM868 is priced around $5 USD for sample quantities
SIM808 is priced around $8 USD for sample quantities
Which module should you choose ?
Handsdown, SIM868 is the winner here !
If you require better GPS reception, Smaller form factor, Low cost solution then you should chose SIM868 module.
Here is one of the image of the tracking device i designed using SIM808.
From my experience, I have used SIM808 modules extensively and i can vouch for it as it's a very good module and has very good and accurate GPS positioning system suitable for any kind of tracking applications.
SIM868 is newer and I am definitely looking to use it in my next design as it looks very promising.