Hardware wise, most Terasic boards as the DE1 are great. Their price is usually very good as well. Documentation and provided samples, however, are not really so great at all.
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I am also hesitating between DE1 board and the Cyclone III starter kit. Can someone tell me which one is the most flexible and can be easily extended?
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Differences are pretty obvious, I think. DE1 is a multimedia board, it comes with several "plug-in" connectors and interfaces, such as VGA, PS/2, Audio, SD card, etc. The CIII starket Kit has none of them. But on the other hand, it has a newer and bigger FPGA, and more RAM and FLASH.
The DE1 expansion connectors are standard 40-pin IDE. The CIII kit has an HSMC connector.