I have came across these SIM modules on an online electronic shop. My primary target is just to receive short text messages from other subscribers. But then this question about these three (if there is anything apart from these, those too) modules. What are the primary differences between these GSM modules?
SIM900 is a quad band modem being able to operate in 850,900,1800,1900 MHz bands and offers improved GPRS functionalities useful in web enabled applications. SIM300 is a triband GSM modem being able to operate only in 900,1800,1900MHz band. SIM900 and the SIM300 modem operate from 3.4V to 4.5V supply range. Same AT commands used for call/sms in SIM300 can be used with the SIM900 modem.
SIM900 is quad band,SIM300 is triband and SIM900A is dual band GSM modem. This makes difference in their operating frequencies. The more the frequency of operation is, the better the functionalities are.
If you need it just for SMS, then I feel SIM300 is a good option and is comparatively cheaper also. It does not make much difference if you will buy SIM300 or SIM900, if you are considering it to be used for SMS services across the world. SIM900A is a good option for use in Asia only.