the billion has SNR tweaking built into it - although you dont need it cos your line is capable of 20Mb anyway, but it has it anyway - it works with MER too with latest beta firmware so you would be protected that way, I will probably get one myself later when I can afford one...
the billion is the one the hardcore users seem to go for... and DMT Tool works on it anyway cos the chip is 6358 but like I say it has SNR Tweaking built in
it is also Cable and FTTP ready with an eWAN port - this router can be used on cable or fibre, it has Gigabit 1000Mbps LAN ports and IPv6 ready - it really is the one too go for, I only got the 834N cos I only had £10 to spend hehe - the Billion BiPAC 7800n sells for about £70-£80 on eBay
if your getn an N class router you need N devices to make use of it, if any of your devices are G then it will not operate in full N mode, a special MIXED MODE applies, some routers just drop back to G i think
the 834N is a poor mans BILLION 7800N hehe - same chip with wireless-N and SNR Tweaking...just minus other features