Oyster card or travel card

It depends on the days of the week you will be there. The travel card didn’t work out for our 7 days (Wednesday to Wednesday) so we just used credit card to tap in and out. Worked great and we hit the daily max.

It's really going to depend. Largely on your expected use of the tube/bus/etc that the Oyster and Travel Card may work on.

If you have a rough idea of itinerary, you can look up fares here: https://tfl.gov.uk/fares/find-fares/tube-and-rail-fares/single-fare-finder
Know that daily fares are capped for pay as you go. I believe it's a max of 8.10 GBP per day come March 5, with a 40.70 GBP weekly cap, on a Monday-Sunday week. If your six days start on say, Wednesday, you could theoretically come close to the weekly cap at 5 days of max for 40.50 GBP, plus the last day at 8.10. That's on a Pay as You Go Oyster account.

A seven day travel card will run you the 40.50.

That assumes maximal Tube/Bus use, or at least up to the daily maximum fare.

If you're going to go from your hotel to a sight, loiter or move without transit around for the day, and then tube home, Pay-as-you-go makes more sense.

I think the only way I'd spring for the 7 day is if my days would cross over, and I'm liable to use maximum each day. Or my expectation is to exceed 5 days of 8.10 GBP.

I always prefer to use my Oyster card, easy to reload at any station & keeps track of your fares throughout the day. I don't like getting my credit card out unnecessarily.

I always used the Oyster Card for the Tube and on buses. If there is left amount at departure, I save that for the next trip.

Why not just use your phone and tap in and out everywhere? Not much difference in cost, if any.

Different forms of travel. Different POVS.

I use an Oyster card because if I lost a Credit or Debit Card OR my phone I’d be livid. Losing an Oyster Card not so much. So like a previous poster I use it and leave London with it to be used upon my return.

Easy peasy.

I prefer to use my Oyster Card with a 7 day travelcard loaded on it as I usually go for 8 days. I keep my card in an easy to scan location so I'm not always opening my bag.

Also, not every phone supports pay technology. My previous phone was Motorola and the model I had didn't have the chip/hardware that allows for phone pay.