This is the same decision I am trying to make. The difference between the 20 & the 40 is the maximum output. According to the specs both pump outputs start at 500gph and the 20 tops out @ 2,000 and the 40 @ 3,000gph. You can also purchase the wireless driver separately which would essentially upgrade a 20 to a 40w.
As for the pro's & con's between the two...I like the Ecotech's because with the 40 you have controllability right out of the box with several different programmed modes to choose from (and of course you can upgrade a 20 to a 40 very easily). I have been reading many positive reviews and experiences from reefers who had problems with the 1st generation models that are now very happy with the new ones. Some people would argue that a drawback of the Ecotech's is the lack of pivotal control of the assembly but the output is so great and dispersed that because of the design and capabilities of the powerhead there really is no need to direct the flow. In addition, the powerhead creates a noticeable undertow to provide good flow along the sand bed/bottom. I have also heard and read very good customer support results from this product. Low energy consumption, battery back up and no motor heat within the tank are more pluses for this product.
I like the Tunze's because they have a reliable, quality product in addition to a well-established name and reputation in the reef hobby. Their support & tech is great and they are releasing improved products with a lower price tag and lower energy consumption. I prefer the controllable streams, over the non-controllable, and even more so a pair with the multi controller and I think the new USB multi controller will be bad @$$. Overall, I think the Ecotech would be less expensive than the Tunze mainly because you have to purchase the multi controller separately and I think it would require less units to achieve optimal flow conditions (but of course this can differ depending on what coral/livestock one is keeping).
These are some of the angles that I am considering and am still undecided but I am leaning more towards the Ecotech at this point...but of course as Brandon pointed out above, you could always use both and decide which one you like better! Hope this helps.
. that's what i'm doing.
