In all honesty, a separate modem and Ethernet WAN router is the best way to go, plus it opens up a whole host of options, such as a hardware firewall option over a router.
However, if you are looking for an all in one type of device you could check out the Draytek 2920 which has an xDSL port AND an Ethernet WAN port so you'd be ready for when FTTx drops to your doorstep.
I've had personal experience with the TG582n and found it very reliable and it does perform well. Although its UI could do with some work
I've also used the P660 series from ZyXEL, however I have found that they sync slightly lower than most others but are solid boxes.
Not had much experience with Billion routers myself, but the reviews and comments I've seen do make them seem worthwhile (maybe I'll try one at some point).
With the ASUS you can (I believe) re-flash it with some Open Source firmware such as DD-WRT which opens up a whole host of options.
Try to stay away from routers that use the TI AR7 chipset, they are known to have some issues with some 21CN exchange hardware. The AR8 is much better, but Broadcom is definitely the stronger chipset - especially as most exchange hardware runs that.
I am NOT affiliated with ANY ISP on thinkbroadband.com, I have used various ISPs and I AM knowledgeable on the subject of broadband.
"Seek and ye shall find"