I've used EasyTrack and they were very good, and cheaper than most (especially GoldTrack who seem to charge a fortune). I think there's more to it than just the time you get on track though, like whether there are instructors available if you want and how much they cost, how good the marshalling is etc.
I've not been on an open pit lane one, but I think I prefer the session based ones. At least you know there are only a certain number of cars on track at once, and most places group cars and drivers of roughly the same speed together to try and reduce traffic problems. Plus you are forced to come in and let your car cool down every so often. I'd be tempted to bomb round all day and break something otherwise ...