I did this once a fortnight for a year - no brainer, Easyjet every single time. And although flights were delayed around me, I had a serious delay just the once... EZ were pretty efficient in their ability to fly on time.

Now, having said that, I was travelling on business, so there were massive advantages to travelling taking 3 hours each way, not an entire day. At the same time, I'd probably only take the train that length of journey if i could get a cheap first class ticket. I'd get the direct, Cross-Country routing, as it goes through a fair amount of variable scenery, and even manages to follow the best bit of the east coast line.