Hi.
Im searching an GPRS/GSM shield for arduino and I saw there are two populars. Sim900 manufactured by IC (http://www.amazon.com/SIM900-GSM-GPRS-Shield-Arduino/dp/B0068EOUEE/ref=sr_1_1?ie=UTF8&qid=1348703158&sr=8-1&keywords=sim900) or SM5100B by Sparkfun (https://www.sparkfun.com/products/9607).
Any of these would work the same way?