the Zyxel has a better WIFI N 2.4Ghz up to 300Mbps while this modem has only 130Mbps
Hopefully the AC 5Ghz is up to 1700Mbps but old device may not support 5Ghz or AC...
The Broadcom chipset 63138 in the DGA4130 is a dual core 1Ghz ARM, which is way faster than the Broadcom 63168 which is a Mips dual core 400Mhz in the Zyxel 8324.
I can't say anything about which one synch better between both modems, but i have a BT HH5A (Lantiq soc) and an HG612 (bcm6368), and my DGA4130 synch 3Mbps higher than my HG612 which sync 2/3Mbps higher than my HH5A.
About the cpu power, the Bcm63168 can do
400 Bogomips per core, while the 63138 can do
1990 Bogomips per core.