The best thing to do is check both models on your line. When I first had FTTC installed in 2012, the ECI modem was far more stable and achieved a far better sync speed, around 3mbps (on a 42mbps line) if I remember. I left it in place for 2 years. I then decided to re-try the HG612, and to my surprise I found that it was now syncing around 1mbps higher than the ECI. Not a huge increase, but you also have the benefit of being able to access line statistics on the HG612. So I stuck with it.
My guess is that cabinet and/or modem firmware updates could've made a difference to the sync speed.