Hi I use the 2820Vn it works really well and has the added benefit of standby 3G when you plug a USB dongle into it!
failover works flawlessly, albeit a little slow compared to ADSL+ but if you have a decent 3G area then it could actually be faster on the 3G!
both will route the VPN tunnel as long as your service provider on the 3G dongle supports it.
good router but I still recommend cisco products over draytek fisher price reputation but if cost is an issue, draytek it is.