Having used both, I would recommend HiveOS.

They are both very alike for the initial setup but HiveOS has quite a few additional features that SMOS is lacking.

Heres a few of those:
- Remote SSH/VNC. (although only on local networks. You could probably do some port forwarding and get this working remotely as well.)
- Monitoring. GPU goes red if temp goes above threshold. Rig goes red if offline. The monitor dashboard helps when you have a few rigs to take care of. You can easily tell if something is wrong with a single glance at the screen.
- Watchdog. Haven't used this much but it can be quite useful to restart the miner or reboot the rig if hashrate is lower than expected.
- Shared access. You can delegate some rights to your rigs to another account. Such as if you hire a tech to help out with your farm, you can give him tech rights. Which would only allow him to do OC, reboots, or OS upgrades. (not allowing him to go into the wallet settings)
- Stats. When something goes wrong, you want to know when and how. The stats page gives you alot of information.
- API. Though I have not gone into this at all. I have not seen SMOS with anything close to API.

Also, if you have 3 rigs or less, HiveOS is free. While you will have to pay for SMOS eventually.