When you're on any custom ROM, an OTA update to the stock ROM should fail. Even if it downloads and you try to install it, it will just error out. I suppose there COULD be an exception to that, but *I* haven't heard of one yet.
As for the ROM comparison, I really can't help you since I've not used either of those ROMs. The only ROM I've used so far (and am still using) is CleanROM. The Dev on that has moved on to other things, so it's not being updated. That's OK with me because it does everything I want a ROM to do.
1) it works.
2) it has good battery life.
Yes, my needs are few...
Of the two you mentioned, I would be more tempted to go with Synergy. Over the years I've found that heavily modified ROMs TEND to be the ones with more problems. Which is NOT to say that I think Hyperdrive is a bad ROM, many folks in that thread swear it's the best they've ever used. It's just a personal preference for me.