I have an EMS that's built solid. It's not in use because I broke the tank it was going to go on... (sigh don't ask). I have a eshopps M overflow box that works... but the water falling over the teeth is pretty loud if you push any real flow through your return pump... plus you will almost be guaranteed to see the waterline if you use it on a rimmed tank.
I've read more bad than good on the synergy, but I have no personal experience with it so take that for what it is...
I've read good things about modular marine, other than it can take a little bit to get your overflow. Took me several weeks to get my EMS in, but I did order it during peak holiday season. I'll also add that it was more of the fault of USPS as it took them 9 days to deliver a package that was like a 6 hour drive away...