What is the best fare card for traveling in London?

My husband and I are going to the UK in May. I have been looking online for travel info and there's so much and it's pretty confusing- I want to narrow things down a bit. Basically, we need to get from Heath-row to Colchester, Es. For 3 days we'll need to go from Colchester to London (several stops in London) and back again, and the last 2 days, just gallivanting about London, and back to Heath-row. So - should I get the Oyster card, the travel card, or the Pay As You go? I got the impression that they're all a little different, and I'm not sure which one to get, and if I actually have to get it in advance or not.. I'm guessing we'll just take the Tube around in London, but we'll need to catch a bus or something from Heath-row to Colchester and from Colchester to London....Any help would be greatly appreciated!