I have not tried the spider yet but have quite a few pairs of the prima xc shorts. I have tried desoto and other brands and nothing has come close to kiwami- the fabric and fit is amazing. I sometimes even wear them just for running (I know odd) since they are so comfortable. I found their sizing chart to be pretty accurate as well.

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Andy Mullen
Team Zoot

The spider line has extra printed on compression for certain areas depending on which garment. There is quad and hamstring compression, ab support on the short distance suit, and bonded materials in other areas. It is quite a bit more expensive to manufacture, and is comparable to Assos quality. I have to say from a few years of experience that KiWAMi definitely more than makes up for the purchase price with quality and extreme durability. I have a few items that are on their 4th year or more.
Another plus is that Andre can customize your gear with any names, logos or printing individually at a very reasonable cost.
Perhaps he'll chime in with anything I missed.