AS5350 ROCKS!!! The main difference is the Nextport Universal ports of the AS5350 can handle both voice and data calls on every port. An AS5300 will have to have both Mica modems for data calls and voice ports for voice calls. The are specialized and only have one function. The Nextport Universal ports of the AS5350 actually download the modem firmware to the ports during trainup if the incomming call is a modem call. So they are multi functional. We traded our AS3640 for an AS5350 and cannot be happier. I have all the same functionality as before, but I have a voice gateway also. It also does true voice switching that the 3600 series cannot. If you are looking for a platform to take you innto the converged voice/data era, then get the 5350. If you are looking for a used system that can handle tons of incoming calls and a pretty cheap price, go on ebay and get the AS5300. Keep in mind that it has been end of lifed (pretty sure about that).
It is what it is!!
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A+, Net+, I-Net+, Certified Web Master, MCP, MCSA, MCSE, CCNA, CCDA, and few others (I got bored one day)