I have been to both countries - Namibia was in 2013 as part of an overland tour that started in Cape Town, traveled through Namibia, and Botswana and ended in Victoria Falls, Zimbabwe. Here is the latest version of that trip which takes you up the western cape, through Namibia including stops at Swakopmund, Soussevlei, Namib/Nakluft NP, Etosha NP, Windhoek before crossing into Botswana. We got to climb Dune 45 and also see the Fish River canyon. Stayed at two different watering holes in Etosha. This provides great wild life viewing since the camps are enclosed and the animals are outside. These watering holes were lit by lights which enabled one to go and sit and view the animals even in the middle of the nigh
https://nomadtours.co.za/this-year/cape-town-to-victoria-falls/
Uganda was in 2017 which was a self organized trip doing almost a full circuit around lake Victoria - starting in Kampala and then traveling through Rwanda, Tanzania, and finishing up in Nairobi. I organized the Uganda portion through Red Chilli Tours. I wasn't interested in the Gorillas - so had them pick me up in Kampala,
- spent a few days in: Murchison Falls NP (this is where the Nile river is squeezed into a cleft that is only about 50 ft wide which results in a thunderous waterfall) - great wildlife viewing - including leopards but didn't see lions IIRC. Took a boat cruise on the Nile to get close to the falls
- next traveled south and spent 1 night in Fort Portal to break up the long drive to Queen Elizabeth NP. Along the way stopped in one of the forest patches to see a troop of chimpanzees
- Crossed the Equator (took the obligatory picture) and spent a couple of days in QENP - again, great wildlife including lions and leopards. Did an afternoon boatride on Lake George. The lodge we stayed at was on a bluff overlooking the lake and you could see warthogs, giant forest pigs, hippos, etc. just sitting on your front porch
Points to note about Uganda - most of the trips seem to start and end in Kampala - we organized ours to drop us at the Rwandan border and then made our own way to Kigali (short 2 hr bus ride from the border). We enjoy the afternoon/evening boatrides and never miss a chance to do this - it is a relaxed way to see wildlife (did one of these in Chobe NP in Botswana too). Also, none of the 2 NP we went to have rhinos though there is a rehabilitation center that you can visit if this is of interest to you.
The key is that the two countries are quite different as noted by Mary above. Namibia is almost all desert which means the animals are highly concentrated around the watering holes in Etosha. Uganda is forested lying along the Great rift valley with some of the large lakes and the mighty Nile flowing through. You need to decide what it is you prefer. Also, if you are mainly going for wildlife, then timing will be important. In general, if you go during the rains or after, then the animals will be spread out since there is a greater abundance of food and water. Hence, most of the recommendations will be to try and go during the dry season where the animals are more concentrated. We traveled through Namibia in early June and were in Uganda in late July