The Spartan-6 was introduced by Xilinx in 2009 and the follow on Spartan-7 in 2015. As semiconductor fabs move to newer process nodes older processes become less cost effective and production volumes decrease. Due to current shortages of Spartan-6 parts, Xilinx is encouraging migration of designs to the newer Series 7 FPGAs.
Xilinx has done a great job of documenting differences between the Spartan-6 and Spartan-7 FPGAs in order facilitate that migration. The two best references I've seen are the white paper and webinar, both done with Adam Taylor from Andiuvo Engineering.
Webinar Mastering the Migration Journey from Spartan-6 FPGAs
I'll try to highlight what I learned from those resources.
Hardware differences
A significant change is that the Series 7 FPGAs are built with the next generation process node moving down from 45nm to 28nm. This enables higher performance and higher circuit density and lower power. The Spartan-7 has 2X the relative performance per watt than the Spartan-6 as shown in the chart below. The chart also shows improved memory speed from DDR3-800 to DDR3-1066.
The Spartan-7 does have some tradeoffs in a smaller Max I/O count (400 vs 576) and the lack of transceiver cores (Gb SerDes and PCIe). If these features are required, then the design needs to be implemented using Artix-7 FPGAs.
There are also a number of significant architectural changes. These are listed in the following table.
Toolchain differences
There are also significant improvements in the development toolchain between Spartan-6 and Spartan-7.
The Spartan-6 uses Xilinx ISE for hardware development and Xilinx SDK for software development. The Spartan-7 devices are using the newer Vivado tool for hardware and Vitis for software. There is a huge improvement in design efficiency gained by using these new tools. Vivado allows working with pure RTL designs, but it also leverages a large IP library managed using the Vivado IP integrator. On the software side, Vitis is a unified software platform that provides a high level framework with open-source libraries that have been optimized for Xilinx FPGAs.
Summary
Spartan-6 devices are nearing end-of-life (EOL) and should not be considered for new designs, but the newer generation Spartan-7 device capabilities and toolchain should allow the design migration from legacy parts. The specific IP and IO requirement utilized in the Spartan-6 design could require using a different 7 Series part other than the Spartan-7.
For new designs, the Spartan-7 is a clear choice due to higher performance, power efficiency, and improved toolchain (with exceptions requiring other 7 Series parts).