Both are Serrasalmus species. Both have tendency to show sometimes extreme skittish behavior when juveniles so keep that in mind when deciding.
Like stated before it depends on what look suits your liking and what tank size you're working with.
Spilos are much more colorful and will be able to be kept in a standard 55 gallon tank.
Rhombeus in terms of color are much more drab but will grow, with a lot of time invested, into larger predators (with a more appealing shape IMO) requiring at least a standard 75 gallon. Though a 48" x 24" would be the better choice.