Both of these restaurants are good, and you should choose based on where you are, and which kind of cuisine you prefer. The recent change of
Tusker House to a buffet with some African-inspired entrees has made it a fantastic destination, and
Mama Melrose's italian cuisine far outperforms the campy decor.
But if forced to choose, I would go to Tusker House. The variety is great, and the buffet provides both some exciting new twists on traditional foods with an African flair, but plenty of classics for those who do not want to venture too far afield.
By the way, you can now see menus for Mama Melrose's on the web page that I have linked above, so that could help you decide as well.
I hope this helps. Please feel free to ask me any follow-up questions from my bio page.
Have a delicious vacation!