As of 2016, go with Justworks.
Zenefits is a great system but it's a framework, not a ready-to-go solution. What Zenefits provides is free software that can integrate with various HR service providers and manage everything in a single console - but you still have to select all those providers yourself, including payroll, benefits, compliance, taxes, etc.
If you want the control of managing exactly which company does each service then Zenefits can work to tie everything together into one system but it's still a lot of setup and various subscriptions to handle.
Justworks is a different model where they act as a PEO (professional employer organization) and HR agency all in one so everything from onboarding to taxes to benefits is handled by them as a single entity. You only have to deal with a single vendor for everything and you'll know the same team is handling all of the separate but important details. It's completely seamless, their online UI is fast and works great, and most importantly, they save a lot of time and effort.
As of Feb 2016 - Zenefits has also been in the news for issues of selling health insurance without proper licensing in California and Washington. This might or might not matter but isn't a great sign for an HR company.