Does anyone have an opinion on whether one should buy a Visitor Oyster Card or just a regular one. I believe that the regular one gives a refund upon return. I did not see that same option with the Visitor Card. Is there any other difference that I should know about?

There is really no reason to pre-buy an oyster - wait til you are London and buy a regular run-of-the-mill Oyster.

Never having had to buy one, I don't know, but I assume there might be some advantage you're depending on it to get to where you're staying and don't want tto queue up with everyone else to get one when you arrive.

A lot depends on when and where you're arriving. If you're coming into Gatwick, you can't AFAIK get one until you get into central London anyway. If you're taking a car service or taxi to a hotel, you might as well go to the nearest tube station or a local corner shop near where you're staying.

There is absotively posilutely no advantage to buying an oyster card before arrival in London. You buy it the first time you use the tube whether it bew at Heathrow, Victoria, Paddington or St. Pancras (the usual arri;val points although there are, of course, others) when you descend into the tube station. And most of the machines that sell them and the booking offices take credit cards (even the antiquated American crfedit cards you might have if you're from the USA).

Visitor Oyster cards have a non-refundable deposit. You also cannot add 7 day travelcards to them.

As already stated, there is no reason to pre-buy. It takes minutes to buy.

One tiny difference may be that regular Oyster cards go "inactive" after 24 months (?) of non-use and need to be reactivated, apparently by transit staff.

The Oyster for visitors, I believe, never goes inactive so this may be a very small consideration if you plan to use your Oyster for repeated but infrequent visits.