I've been using the DSL-AC68U for a couple of years now and whilst initially there were a couple of issues with the firmware they were quickly fixed by Asus.
Since then it's been absolutely rock solid.
As for the DSL-AC88U I've no experience but as with the AC68U there were some initial firmware issues I believe but if I was replacing my AC68U & was looking for a G.Fast modem/router I'd go for it.
I'm not nessacarily fussed about G.Fast support as no doubt by the time it arrives I'll either (A) be replacing the whole network anyway or (B) not be eligable for it anyway due to line length/quality.
What I would like is though a decent stable VDSL modem that I can view stats on (not nessacarily the full telnet stats like the HG612) and that is slightly more modern and may give me better speeds and stability that may reduce the interleave I have set.
I was looking at the DSL-AC68u too as it now supports the full AIMesh which means I can link it to my exising RT-AC68u for a full mesh network. The DL-AC68u seems to get more updates as the modem is a seperate unit bolted to an existing RT-AC68u, wheras the 88u the modem is part of the whole SOC and as such it seems to be quite far behind in getting some of the features my older 68u has. I dont doubt they will come eventually but the firmware development for it seems very slow and thats a worry especially as some have reported flaky wifi and other issues that will likely be ironed out in firmware but may be months down the road if at all.
Glad your experience with the DSL-AC68u was positive though, has it improved speed and stability for you at all?
One of the reasons I was looking at the 88u is one professional review site said it managed to boost their connection by 5Mb which is rather impressive.
I've never used a HomeHub to compare against, my only Infinity setup has been using the HG612 with an Asus RT-N66U before upgrading to the DSL-AC68U. Can't really comment on sync speed comparisons between the 2 combos as my line conditions have changed over the past few years but I can state the the AC68U has been rock solid , as I've already said, as has my son's which he's had for maybe 18months.
Stability is often a factor of the line condition rather than the modem/router as I'm sure you know , unless you're using a HomeHub which can be up & down like a yo-yo.
I have to admit I quite like the DSL-AC68u as its just my RT but with a DSL modem bolted on. I may look into it, I'm gonna take my time and see if the 88u firmware gets a bit better. I'm in no hurry as the Openreach modem works fine for now, that is unless there are other VDSL modems that are much cheaper that will do the job with my RT-AC68u.
You could look at the Netgear DM200 at about £35 for a standalone modem.
My BT Infinty2 has been fine over the last few years with a Home Hub 5A. However the max data rate has slowly been erroded from 88mbps to 74mbps due to, I assume, crosstalkers.
I recently got a Zyxel VMG8924-10A and installed it this morning. My connection has improved by 5mbps, which I'm impressed with and max data rate is now dead on 80mbps and actual is 79.70mbps. Speedtests show this improvement too. For a few quid off ebay, I'm over the moon and it's lovely router to use. Wifi seems on a par with the HH5A.
I believe too it runs dslstats, so I'll look into that as well.
I've heard good things about the ZyXel modems, by all accounts they are a good drop in for the Openreach modem. Shove it in bridge mode and put it in front of a more powerful router. I read that one of the ZyXels boosted a connection by around 5Mb/s over the HG612 due to the much newer better chipset.
My have to look on the Bay to see if any going cheap.