There are plenty of differences between GHQ and FOW, both I reckon make for a very enjoyable game in their own right. Unit cohesion is the big thing in GHQ, where you have to roll under your unit's cohesiveness to be able to move them. I found this frustrating at times but it did feel right somehow. With FOW all units can move regardless of whether they begin as reluctant or fearless (or anything in between). It's only when units begin to take losses that these ratings come into play, like any traditional set of morale rules.
The biggest difference between the two sets of rules has to be artillery. Both sets require observers (as they should do). With GQH you plot your artillery fire for several turns at the start of the game depending on the abilities of your forces gunners. You can only target areas that your observers can see. In the following turns you continue to plot bombardments, one per turn, but have to be mindful that you are plotting several turns ahead so as not to end up with a 'Blue on Blue' situation (which I of course did manage to do a few times
). FOW's artillery is simpler, observers see a target, you place the artillery marker and roll to range in, getting three attempts to do so (four if mortars as they are allowed to re-roll their first attempt). If you don't range in on the first attempt any subsequent success means the effectiveness of the strike will be lessened.
I like both rules sets very much, they have a lot to recommend them. GHQ's artillery system is a joy, I loved trying to guess where my targets could/would be. The cohesion, while a pain at times works out fine, especially for campaign games as units are chewed up in the meat grinder of continuous battles. However I feel that where FOW beats GHQ and beats it hands down at that, is the speed of the game. There is no need to keep any notes (artillery) you just get on with it and it flows so well too. It may not win any awards in the ultra realism stakes but it's a helluva lot of fun to play.
PaulMc