I didn’t really find my ZyXEL to be faster than the previous modems I had been using, the DLink DSL-320B-Z1; in fact the ZyXELs’ upstream was slightly slower, which is annoying, until very recently when the upstream speeds have all increased post lightning damage. But the ZyXELs are more stable, as the Dlinks would go down to very low downstream SNRM values, such as 1.8dB or even less, rather than sticking to the assigned 3dB d/s target. With the Dlinks, wandering off on a voyage all the way down to 0.8 dB downstream without authorisation was not unheard of. But anyway, if the d/s SNRM value were adjusted to be the same, then the ZyXEL modems would be faster. The other advantage of the ZyXEL is its support for the Broadcom PhyR - proprietary G.INP - L2 retx protocol for ADSL2. They’re just extremely stable. Some times they do get amazing downstream sync rates on ADSL2.
I have 7km long ADSL2 lines, your circumstances are rather different from mine though so the ZyXELs might be better, might be worse.