I'd say it's still actually quicker from Euston to Kings Cross on the bus, despite it often getting stuck coming out of Euston bus station. Taking in to account the distance you have to walk at each of the tube stations.

You're best off getting an Oyster card if you're going down again. £1.40 per single fare on the bus, with a price cap of £4.40 a day. There are no peak/off peak or zones on buses.

DLR is the same fare structure as the Tube, so you have zones/peak/off-peak, and would be paying about £7.00 for your daily price cap in zones 1 & 2. I'll never begin to understand the zones structure, so I've just got auto-topup on my Oyster. Life saver