The best choice is a fully baffled flat tank. This maintains a low centre of gravity which will help with road-holding, MPG, tyre & suspension wear etc. The baffling needs to be of the multi-pocket type (as mentioned above) as generally the structural baffles are not enough. One of the best ways is to get drainage pipe of any type and cut lengths just short enough to be dropped into the lid of the tank and to stand upright in the tank. Simply slide the first one in and fit another - carry on doing so until the tank is full.
A large upright tank is simply not as safe and will adversely affect your vehicles handing and performance. It also will perform far worse in a crash situation. When carrying out crash testing of our own tanks we found that the lower the tank the less effort it took to hold it in place - the result is that we now have the lowest profile of any crash-tested tank and restraint system.
The other advantage is that there is a good amount of room above the tank in most situations to fit other goods if you need to transport stuff in your van at the weekend.