For rotating Bose-Einstein condensates, the occurrence of vortices has been much discussed in the Gross-Pitaevskii approach. Here, we study vortex formation in finite systems with few particles for weak and repulsive two-body interactions, by numerical diagonalization. Vortex formation is surprisingly similar for both (spinless) bosonic and fermionic particles. Breaking the rotational invariance of the Hamiltonian of the system reveals the vortex patterns. A transformation from a many-fermion to a many-boson state is proposed.