It depends on a number of things really.
1. What is your water chemistry?
The majority of American (Central or South) require a slightly acidic PH and usually water that is soft - medium hardness. Rift Lake Cichlids that come from the Malawi, Victoria or Tanganyika do best in water with a higher PH of around 8-9. They also prefer medium - hard water. Other African Cichlids such as Pelvicachromis, Anomalochromis, Nanochromis etc are very similar to the American Cichlids with regards to water chemistry.
2. Filtration?
Ensure you have very good filtration if you're choosing a Rift Lake setup. You will need to heavily stock this kind of setup to ensure aggression is spread throughout the whole colony. Also, with the medium - larger American Cichlids you will also need strong filtration as their bio-load is massive. The smaller Dwarf Cichlids such as Apistogramma, Laetacara, Pelvicachromis will obviously be lighter on the waste they provide. These kind of Cichlids also allow you to stock a more mixed tank that can include fish like the smaller shoaling Tetra's. Corydoras etc.
3. Maintenance?
My Malawi tank is probably the easiest tank i have to maintain. There is quite literally nothing to do other than a quick wipe of the glass everytime i do a water change. Usually, due to the nature of stocking with this kind of tank a weekly 50% water change is needed to keep them looking at their best. If large water changes are an issue for you then consider this when choosing your fish. If choosing the smaller Cichlid route then ofter plants are used also. These need maitenance, especially if you go with fast growing stem plants. Java Ferns and Anubias are low maintenance plants but can often attract algae due to this.
4. Tank size?
55g is often considered the smallest tank for Malawi's. There are the smaller varieties such as Saulosi and Demasoni, but also bare this in mind when looking at fish. If some larger varieties take your fancy then 75g will be a far better option. 75g will also allow you to stock some of the medium American fish such as Jack Dempseys and Green Terrors etc.
5. Amount of fish?
If you want a super active tank then a Malawi/Victoria type setup would probably be the best option. American Cichlids need far more space due to them occupying small territories per fish/pair.
6. Cost?
Also consider this when planning your tank. Again, due to the amount of fish you need to stock a Malawi type tank it can become rather expensive. Often finding a local breeder is a far better way to stock a Malawi tank. Tanganyikans often demand a higher price tag due to them not being as popular and slightly rarer overall. American Cichlids are usually the cheapest fish available and due to the numbers you will be able to stock will more than likely work out the cheapest option.
7. Tank Setup?
What appeals to you the most? A Rift Lake tank will usually be made up of predominantly rocks. On the other hand an American Cichlid tank will usually contain less rocks and some wood. As i mentioned, plants are completely up to you, but they do fit in well with small American Cichlid setups.
Here are a couple of my tanks to give you some kind of idea of how different they can look and that may swing you towards one or the other.
Malawi Tank by
David Raynham, on Flickr
Apisto, Tetra and Platy Tank by
David Raynham, on Flickr
