I have a Billion 8800AXL (the 5GHz version of the 8800NL) and a ZyXel VMG8924. The Billion is absolutely rock solid on my line and just works. I don't have to reset the target SNRM tweaks (ADSL) as it has the functionality on board and it will also increase sync rate, automatically, when the line conditions become more favourable it would seem. It also gives an overall more snappy experience when using the web, than the ZyXel.
The ZyXel has been a mixed bag but that almost exclusively seems down to the various FW versions that have been and gone. On v15 I used to lose WLAN access from any device when my wife's iPhone was using the WiFi. With v18 & v19 I'd just lose access to the web or certain websites, despite the sync with the exchange holding firm. v16 seems to be the sweet spot, without wanting to influence my luck. My experiences seem to be unique though it must be said.
The ZyXel seems to sync at the highest rate (down). I've seen it sync at 19Mb/s and hold it for a week or so, until I reset the connection. The most the Billion syncs at is 17.6Mb/s. However, I'm willing to concede that could be coincidence with the environmentals that are present at the time, as the Zyxel is currently syncing at 17.3Mb/s. I prefer the interface of the ZyXel, specifically where you can have thumbnails with customisable icons and names, for the devices using your network. Actually, that's the only reason I prefer the ZyXel's UI over the Billion's.
All that waffle aside, you'll do well with either the Billion or the ZyXel.
Paul