I use Virgin cable at home so don't need the modem part of a 2860ac. The 2925ac looks the same spec except for the missing ADSL/VDSL stuff. So is there any advantage in buying one or the other? The 2860ac presumably would do the same job as the 2925ac acting as a pure router downstream of the Virgin modem. Pricing seems pretty similar. I'm just wondering if the presence of the modem components of the 2860ac gives more risk of failure or shorter life expectancy even if not actually being used.

I am buying mainly because I want to isolate parts of the LAN so I don't get caught out by a botnet attacking my PC or NAS if I also have other "smart" equipment connected such as smart TVs or Virgin's Tivo (or if my wife buys a smart appliance god forbid). Also buying for the better quality of the kit vs a consumer oriented router.

I picked up a cheap 2925 from ebay (feed mine from a 130), which was a significantly cheaper solution than buying a 2860

The specs are very similar, 2925 has 2 x 1Gb WAN sockets, the 2860 just the 1.
2925 has one less LAN socket (5 vs 6)

http://www.draytek.co.uk/products/comparison

In the end I bought a 2860 as the prices were similar and I use this at "work" (in quotes because it's a voluntary thing for my club).