Vlad, you are mostly right in your opinions...
25 degrees delta is not much, but it seems Atik 4xx reaches between 28-29 delta (at least my and my friends cameras do reach up to 29deg delta). Whether it is enough or not depends on where you live. If the ambient temperature at night is below +20deg C then you are OK. For warmer places on the World it would be perfect if Atik provides some cooling enhancer
BTW SX also has small delta, but with "cooling enhnancer" - that is in fact just a fan (otherwise the camera body heats too much) it reaches around 30 delta as well as Atik.
SX is more noisy then Atik. SX provides you a way how to collimate the chip tilt/aligmnent which might be good, but also might be bad as it's another thing you have to worry about and it may not stay permanently perfect. Atik on the other side also (at least I have found out) had some "problems" with alignment, but I was told (when I reported to Atik how to make their products even better and uncompromised - as I like Atik) they work on a new process how to make this even more precise then now. SX has backfocus I think 17mm? or something big while Atik has only 13mm which is just perfect
Best is to use SX accessory (filter wheel for instance) with an Atik camera
http://blog.astrofotky.cz/pavelpech/?page_id=782One more remark from my tests - the threashold temperature (from which you may not ever notice lower noise in your images) that you should cool down to is around -8 deg C. This seems to be the breakpoint. Anything cooler than this brings only very small benefit. Therefore I set mine to -10 for all year round (except of Winter times when I shoot in -15 ambient).
all best
Pavel