Decklink cards DO NOT have hardware scalars. If you ever plan on ingesting signal to DVCProHD, you will experience lossiness. In addition, the lack of hardware scalar and tie in to RTExtreme means that a Decklink card is not doing any math scaling out your images for output or monitoring. This literally equates to less real time using DVCProHD (even if you captured using a P2 card).
AJA’s cards all have hardware scalars on the card. This frees up processor time by handling all scaling of non full raster codecs (HDV, DVCProHD, XDCAM). You can argue that you don’t need the real time performance on output, but I would think that when doing a capture/transcode to DVCProHD, the quality of your ingest would matter.
There are more reasons, but I would think if DVCProHD workflows are important to you, this would be a big sticking point.
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-Mike