We just got back from Zambia this past weekend, and figured I might add to this for any future reference. We split the trip between South Luangwa and Lower Zambezi, and picked Zambia for some of the same reasons noted by other folks above, mainly it is less crowded, and seemed to provide more of an opportunity to "drive at your own pace" so to speak. We were not disappointed, and in fact, the trip exceeded expectations in every aspect.
In Luangwa we stayed at Lion Camp, which is about 90 minutes away from Flatdogs and into the park's inner area. Lion Camp has 10 tents/rooms, so on the smaller side, and typically has about 15 guests at a given time. The units are all built on a platform as the local elephant population tends to hit the watering hole and munch on the trees in the camp and an occasional hippo or two wanders through at night. The guided rides were split into 3 vehicles so we usually had 4 or 5 people in a vehicle and mostly had an area to ourselves unless we ran across the local lion pride or a leopard, in which case the guides would radio their colleagues so everyone got a chance to see the sights.
The staff at Lion Camp was absolutely amazing - guides are extremely professional, patient, and will work out routes based on what you have a preference for on a given day. The camp was renovated in 2010 and a new team was brought in at about the same time - each of the three guides has gone through a full certification program of 3 years, their English is outstanding, and the camp staff is great as well. The typical day is a quick breakfast followed by a 6 AM day ride (or combo ride / walk), back for lunch and a siesta, 3:30 Tea, and an afternoon / night ride between 4 - 7:30 or so depending on the local action. Dinner at 8 - you can see that the greatest risk here is gaining weight.
From Luangwa we headed to Sausage Tree on the Lower Zambezi. At this point I do want to note that this trip was a 25th anniversary celebration, so Sausage Tree was our splurge. The lodge is on the river, with only 6 units, (with very cool outdoor bathroom / showers with concrete floor and privacy walls but no roof at all) and every couple gets their own guide and vehicle. The daily schedule was not much different from Lion Tree, and you get an additional choice of a canoe guide on the river or a motorized boat guide, but the service includes room delivery for your morning coffee, a menu selection for dinner, and each guest gets lunch once during their stay in the river itself on a sandbar with the water about 12 - 18 inches high. Being on the river, getting to the lodge included a flight from Lusaka to Jeki, a ride to the river, and a boat transfer to the camp. As a side note, the Jeki airport includes a dirt runway and a guy with a piece of paper writing arrival names on a piece of paper. Sausage Tree has a sister lodge within a quarter mile, Potato Bush Camp, which is more of a typical lodge.
The guides here were good, but I would say that they were not as good as the team at Lion Camp in terms of knowledge and ability to spot game. To some degree that may be partially a result of the Zambezi area having a higher concentration of thicker brush, so there are more hiding spots.
In both spots you see more or less the same game (no rhinos in Zambia for those who are planning a trip, and no giraffes in Zambezi but plenty in Luangwa). Zambia is blessed with diversity of wildlife so you will see everything but there are no large quantities of any given animal traveling in a group. If your preference is to see hundreds or thousands of animals in a migration pattern, this is not the spot. What you do get is a chance to see a cross section of large animals (4 of the big 5), dozens of bird types ranging from eagles to 4 inch miniature white owls and baobab trees with trunks in the 30 foot range before they split into large branch sections. As it's less crowded, you get a lot of flexibility - for example one evening in Zambezi we caught the cubs and 2 - 3 year old lions trying to hunt. The guides (there were 2 vehicles from the camp together) basically parked the cars in the middle of the pack and turned off the lights. You could hear the impala hooves and the warning sounds they made as well as the lions running after them. No impalas made the menu that night, but they did get a warthog.
To close, I would highly recommend Zambia for anyone heading out for a safari, and for those who want an experience with fewer people and vehicles around, would absolutely recommend Lion Camp in South Luangwa.