The answer to your question depends on which country you are in and the specifics of the given Masters programs you refer to (different universities have different requirements for biostatistics or bioinformatics).
Assuming that you are based in the US, I would think that either degree will have good employment prospects, with an edge for biostatistics because of the added background in statistical analysis and modelling would give you added flexibility in pursuing a career in other related work where statistical analysis is required.