The iPhone 4S appears to perform better in certain areas on different carrier networks, according to research released today from device testing firm Metrico.

For the analysis, Metrico performed more than 21,000 web page downloads, ran more than 8,000 data download/upload tests while mobile and while stationary, and generated approximately 6,000 voice calls, using the company’s global measurement system platform.

The results? According to Metrico, call success rate, or the ability to make and hold calls successfully, were slightly better for the Verizon iPhone 4S than for AT&T and Sprint versions.

Sprint’s iPhone 4S performs better than the other carriers in live network voice quality on the uplink, while AT&T’s iPhone 4S takes the lead in live network voice quality on the downlink. However, Metrico said that the two carriers’ results are comparable.

Across the three carriers, when compared to 3G smartphones measured in each carrier’s portfolio, Metrico says the 4S is among the top performers for speech quality in noise; average in speech quality over Bluetooth; and a below average performer in speakerphone speech quality.

Perhaps because of AT&T’s HSPA+ network, the iPhone 4S outperforms its Verizon and Sprint counterparts in data download and upload speeds. Moreover, the AT&T iPhone 4S outperforms the iPhone 4 in download speeds by a considerable margin.

Overall, the iPhone 4S outperforms more than 70 percent of competing 3G smartphone models tested on the three networks, according to Metrico. The iPhone 4S apparently gets the jobs done. The phone’s success rate of completing tasks on all three carrier’s networks was nearly 100 percent in stationary downloads, stationary uploads and web page loads.

Metrico calls the improvement gains in stationary download reliability over the iPhone 4

“dramatic” for AT&T and Verizon.

Amit Malhotra, vice president of marketing for Metrico, said the results prove that the iPhone 4S is a definite improvement over the iPhone 4, but the testing also highlights strengths and weaknesses across carrier networks.

“It’s always very interesting to us, because this kind of happened when we tested the iPhone 4,” Malhotra said. “We find that each carrier has its own strengths. So Sprint does really well in voice quality. Verizon’s doing better than the other two in call performance and AT&T’s doing better than the other two in data.”

But aside from the differences in how the 4S performed across the major carriers, Apple’s new phone also showed improvements over its predecessor. “The data reliability is particularly striking, going from when we measured the iPhone 4, and it had like 90 percent reliability rate, which is still pretty good, to near 100 percent for the 3 carriers.”