I've used both and find I prefer ExpertFlyer. The convenience of having it be web based is hard to ignore - especially since KVS has a fairly onerous non-automated activation scheme. I own a bunch of computers and switch between them depending on what I'm doing (I select a size of laptop for each trip, Goldilocks-style )... because KVS doesn't have automatic activation, I was stuck waiting a day or so for manual processing of my e-mail request for a new key (which caused me to actually miss out on having data when I needed it.)

Also, ExpertFlyer has very accurate data on the Delta complimentary upgrade buckets, which is invaluable to me. On the other hand, ExpertFlyer doesn't seem to have Delta award data - so if I find myself trying to plan some absurdly complicated RTW ticket, I'll probably end up back with a short term subscription to KVS. (This hasn't been a big deal so far - EF does have Alaska award data, and that's usually what I'm hunting for.)

That siad, you should have access to one or the other if you're serious about maximizing your frequent flier benefits - EF combined with ITA Software is my one-two punch of secret weapons!