The answer to the question is pure opinion since both types will survive for many years and each requires similar attention and care. So here is how each compare in certain factors:
Aesthetics: Single trunks are traditional and what most envision in a domestic yard or park environment. Multi-trunk trees have a more "wild" or "natural" look since this style is common in the woods or forests. Since they are different trees, multi-trunks of the same species may even have different appearances due to unique genetics.
Structure: Singles are straight forward. Most have a main "leader" trunk with side branching. Multi-trunk trees are a bit different since each trunk is a seperate tree grown from a different seed. As the trees get larger the trunks often fuse together (called grafting) and look more like a tree with low branch structures. If you were to cut these down they would show multiple sets of growth rings confirming seperate trees. Structurally, multi-trunks are weaker only because the trunks often lean away from one another. The canopies most often lean as well, which can result in a strange shape should one of the trunks be lost, resulting in "half tree" scenario. This leaning often results in a tree that is wider than it is tall. If selecting multi-trunks for planting, pick trees where each trunk is equal in size and has very little lean to it. This will result in a tree with better shape and strength. As the trees get large, consider having the tree's trunks cabled or braced by an arborist to prevent trunk splitting and preserve the shape of the tree.
Growth: Single trunks grow at a rate determined by the species of tree and the care it gets. Multi-trunks have seperate root systems and since they are close together, they compete for nutrients. This competition results in slower growth rates. Make sure that each trunk is the same species and thus one would not outgrow the other. Since they compete, multi-trunk trees grow 30% to 50% slower than singles trunk trees of the same species. Despite a slower growth rate, these trees can still flourish with attention and care. Ensure they have adequate sun, water and nutrients and they will have no problem growing. Just consider, all things being equal, the multi-trunk trees will be a bit smaller after a given period of growth.
I have removed trees where part of the multi-trunk tree had split away and the owners said, "I wish I hadn't planted a multi-trunk tree." With proper care and carefully placed cables and braces, these trees will last just as long and provide their "wild" beauty as long as their single trunk versions.