This question has been asked over a hundred times, and their is no right answer, it depends on what fits you best. Ill break it down for you.
1. Savannah is cheaper
2. savannahs are a larger creature
3. savannahs are more prolific eaters
4. savannahs are more common
1. Ackies have a better temperment
2. Ackies are more expensive
3. Ackies are excellant beginner monitor
4. they grow to be a much more manageable size
5. Ackies tend to be much hardier
it really depends on temperment and size, while any monitor can become tame with handeling, the ackie will need MUCH less attention than a savannah, and a savannah will almost double a ackies size especially in wieght, these guys turn into fat lazy creatures if not taken care of properly. I own 2 savannahs and a nile, i have friends with ackies, Im happy with my sav's but i like a larger more wild creature, it all depends on you.