I think the A10 route is probably faster if you're in a hurry but it's a main road out of London, so best to hit it early. Some of us did this one last summer and it was fine early on a Saturday morning.
The Barnet route would probably be a calmer/picturesque route to take though.
I'm pretty useless at routes though and I tend to just follow the rest of the group
If it were me I'd do this:-
http://www.bikely.com/maps/bike-path/493922
Out of Waterloo and along The Cut then Union St to Borough High St, over London Bridge.
Bear right at Monument and up the A10 past Liverpool St station.
By Shoreditch High St Tube bicycles (and buses) can go straight on, so you don't need to do that loop down Bethnal Green Road/Arnold Circus.
Blat along the A10, go straight on (right lane) by Bruce Grove station to go onto the A1010.
Up to Waltham Cross (you can go straight on through the shopping centre if you're early enough, I do this if I'm there before 8am).
Carry on along the B176 to Cheshunt.
Left onto Church Lane and cross the A10 (there are lights).
First few lumps as you climb up to Wormley, Brickendon and into Hertford.
Out of Hertford to Bengeo, small section of the not so nice A602.
Whempstead, Burn's Green, Benington, Walkern, Cromer, Rushden, Wallington.
Cross the A505 (quite easy to do), Ashwell, Guilden Morden, Wrestlingworth, Potton, Gamlingay, Waresley, Croxton, Graveley and into Godmanchester and then Huntingdon.
The route up to Ashwell is pretty much the same route I followed on my 3 day London to Edinburgh ride (before doing the London-Edinburgh-London Audax):
http://www.bikely.com/maps/bike-path/266092