Hi Bob, the UC 560 and 540 make ordering & licensing much simpler, offer increased voicemail storage/ports, additional DSP resources, and larger compact flash memory. Also, the 560 can scale up to 104 users.
The UC 520 has not been announced EoS. Keep in mind that the 520 is "traditional" Cisco product, whereas 560/540 are Small Business Pro. This means that 560/540 are ordered only through distribution partners, supported by Small Business Pro Support service (not SMARTnet) via Small Business Support Center (not TAC), and have different discount percentages too.
To find the actual part numbers and additional information on the SBCS & Small Business Pro family, please refer to SBCS Resources for Partners http://www.cisco.com/cisco/web/solutions/small_business/products/voice_conferencing/smart_business_communications_system/index.html-tab-ForPartners (the SBCS and Small Business Pro ordering guides on the "For Partners" tab, specifically) and also the UC 560/540 platform reference guides and data sheets https://www.cisco.com/cisco/web/solutions/small_business/products/voice_conferencing/uc_500_series/index.html-tab-Resources