Hi zznoname. The main thing to understand about EF vs KVS is that KVS just screen scrapes publicly available websites for his award inventory. As such, anyone can get the data for free. ExpertFlyer has direct GDS access, which allows us to get hidden data that isn't available anywhere else, such as AA upgrade inventory. There is another website, AwardNexus.com, which screen scrapes the same Oneworld sources (such as JL.com for CX award availability). AwardNexus has the benefit of being web based (which as ESpen36 points out, can be important), and has a free version for FT members that you should evaluate as well.

The chart that Lux put together is helpful for a features comparison, however the airline comparison is out of date (the up to date list for EF is here: http://www.expertflyer.com/sessionlessClassList.do), it also doesn't include what airline awards AwardNexus shows, which for Oneworld is the same as KVS.

At the end of the day you need to try all three tools to find what is best for you, regardless of the "Out of the two" statements you may read implying otherwise. We think that helping someone to use the best tool for their needs is more important then guiding people to just use our tool. If you have any questions, feel free to email us or PM me directly.