I've only had the opportunity to try QWERKeys
"K Series" so I can't really comment on their "J Series" (aka Cherry profile, same as imsto's).
The K series keycap is just too bulky to use as anything other than the Esc or some isolated R4 spot. It's very thick, durable, high quality and perfect for novelty keys.
imsto's thick PBT keycaps are unique, not just from a thickness perspective. They are PBT dye-subbed, which is a high cost/quality technique. While SP can do something similar and for relatively the same price, they cannot offer other Cherry quality traits, such as: thicker keycap, sharp corners and Cherry font (yet). SP also can't offer a sub-$100 price without a mass order either. imsto on the other hand, you can just purchase directly from him for under $100. One con with imsto is the limited colors he offers at this time.
Now speaking strictly on thickness. The thicker the plastic, the higher "quality" rating it gets it seems. Essentially, there's more plastic making it worth more as a tangible production goods. From a feel perspective, the thicker keycaps also tend to make a different resonating sound. It's a much lower tone on the bottom out, and with clicky switches (i.e. MX Blues) the high pitched click from the actuation is dampened significantly.
Overall, I think the thickness aspect is driven mostly by the original Cherry keycaps, which are notorious for their thickness and higher quality. imsto's new Cherry replica set is very close to original Cherry PBT dyesubs. Take a look here for comparisons:
http://geekhack.org/index.php?topic=34732.0