I am not a power user but I do subscribe to both and can give you some information.

It's not that one tool is clearly better than the other. They both have strengths and weaknesses. EF is better for some tasks, KVS for others. I think automated alerts are where EF truly shines (when they work) but it has many other worthwhile features as well. With KVS I can have a base search and just switch methods to search multiple carriers/alliances very quickly so long as I am looking in a tight range. KVS is supporting more and more POS options on searches which can be helpful. JL for instance has some awards only available with POS Japan and with most search tools you will miss those US->JP POS JP awards. You can book POS JP awards via AA by calling their Japan call center and requesting POS JP. I like using KVS's Awards/VS method to search for DL awards. Searching on the VS website can restrict the origin or destination airports but with the KVS method you can type them in. The limited DL awards I've checked have been cheaper through VS or AF than DL. DL can really gouge you on the miles.

Downsides of KVS are that it mostly accesses your own accounts to search and I have run into more and more cases where you need to answer a captcha before a search proceeds and some carriers will throttle your searches if you do too many too fast (lock you out of searches for a period of time - I'm looking at you QF) which tends to handicap KVS's power search for some carriers. For AS I don't really see KVS having much benefit. The AS website returns queries quickly and makes for usually faster searches on AS than I can get through KVS, especially when considering the AS website returns multiple day availability and has a calendar search that will show you an overview of a month at a time. KVS has to have frequent updates to match changes in the carriers web interface which means you have to update sometimes before you can search (KVS will tell you if you are required to update before starting a search). EF being web based does not require user updates although certain features can break from time to time and EF seems less responsive to fix them than KVS.

Downsides of EF: I think maintenance on it has gone downhill since TPG bought it. I have run in to multiple bugs over the last few months which are not fixed unless I report them and then it may take quite a while for them to fix. Their DL award search was broken for a long time. Their EK award search too which I reported a week ago and is still not fixed. They claim it is a problem with their data provider and all they can do is report it to them and wait for them to fix it. If your provider is not responsive to you you should probably get another provider. I had set an alert for UA inventory on a flight for the summer and the alert never triggered even when the inventory became available. EF's response was that the flight time had changed more than four hours and their system couldn't handle it so they did nothing. They did say they may try to fix this issue in the future. I'm not holding my breath. I had two alerts set for a flight, a schedule alert and an aircraft change alert. The flight was canceled and EF never triggered either of the alerts. When I asked they said that a canceled flight does not qualify as a schedule change or an aircraft change to them which is why they never notified me. Hmm, OK, I asked them what alert to set to notify me of a canceled flight and they say they do not have one -- oops! When I access EF through a VPN it hits me with a captcha for almost every search I do which is annoying as heck.

So, I like both EF and KVS for different reasons and I see enough value in both for me to subscribe to both but depending on what your needs are you may find one or the other more beneficial to you.