I have done some comparisons of ECW versus SID images on my pocket PC, where decompression speed is paramount. The ECW images decompress 6 to 10 times faster than the SID image created at a similar compression rate from the same source file. Not only that, but the ECW images look a little nicer, with fewer compression artifacts. If you have to compress your images and can use ECW, I'd advise it. If speed is what you're after, don't compress your images at all - use GeoTIFF or something similar.
Kenton Williams
