The time of year can be a critical factor in choosing between the 2 destinations. Both are great places to visit from June to November. Tanzania is the wettest from March to May, so if you have set travel dates during this time, Namibia is a better option.
If your travel dates are flexible, another factor to consider is whether you prefer a private safari with a private guide and vehicle for the duration of your safari. The northern parks of Serengeti, Tarangire and Lake Manyara in Tanzania are perfect for a private guided safari as the parks are close together and starting your safari from Kilimanjaro with your own private guide and driving between the parks is a very popular way . on safari in Tanzania. This also makes a safari in Tanzania a better option than Namibia for a family safari, especially if you are traveling with younger children.