Labor or labour, color or colour? This is easy - Shakespeare wrote 'Loves Labors Lost', not 'Loves Labours Lost'. In other words, English of this time spelt '-or' as '-or', not '-our'.

Here is the first version to be printed, the Quarto of 1598:

Already the '-our' was coming into vogue as a French affectation. The 1623 First Folio spelt it '-our':

Clearly 'labor' is older than 'labour', and more economical (what is the point of the the extra 'u'?), so 'labor', 'color' etc win.

Program or programme? Here again, early English usage and economy both favor 'program' (Brits of course favour programme). I have been told that the extra '-me' is again a later French affectation, and worse, it is inconsistent - 'anagram' is nowhere spelt 'anagramme'. So gram and program it is, not gramme or programme.

These first two examples favor American usage over current British. Do any go the other way? Yes - the American habit of ending words either '-ize' or '-ise' at random is inconsistent and hard to remember, compared to British usage of always ending '-ise'. So although Americans criticize and have to memorize, they also advise and televise; Brits on the other hand criticise, memorise, advise and televise. British usage is more consistent, and much easier to remember - so that is what I use.

Usually American usage is more economical, and has more often kept middle English unchanged, and so I usually find myself preferring American English rather than later British derivatives, if there is no other factor: catalog rather than catalogue (what is the extra 'ue' for? Another French import!), draft rather than draught, maneuver rather than manoeuvre, judgment rather than judgement.

Sometimes there is another factor other than economy. I prefer the British 'cheque' (even though it is another French import) rather than American 'check', simply to distinguish the two uses of a similar sounding word; so I check my bank statement for a cheque (an American would check their bank statement for a check).

Economy does not always favor American usage. While British tend to put an extra and useless 'L' before the '-ing' ending (I prefer traveling in America to travelling in Britain), the Americans often insert an extra and useless 'L' before some other endings, so I prefer British enrolment to American enrollment. Logically, I should prefer British skilful over American skillful, but this is a case where American just looks better - probably because the root noun 'skill' itself has two L's.

And I love American 'gotten', the past particple of 'got', which allows for subtle usage. On pay day, both an American and Brit will say 'I got paid', meaning they were given their wages as a matter of course. But if there is some difficulty in getting paid, say, and they only collected their money after some trouble, the American will say triumphantly 'I've gotten paid', whereas a Brit doesn't have a 'got-' word to indicate the process. He will have to add a qualifying adverb to express that getting paid this time was an achievement - 'I got paid eventually'. It is shame that 'gotten' only remains in current British in stock phrases such as 'ill gotten gains'.

So it goes on. What about center versus centre? Is there a defence against defense? Of course one advantage of deliberately adopting mid-Atlantic spelling is that I am free to spell how I like. But in practice the guiding factor must be that writing is understandable and easy to read. If there is a litmus test, it is this: given my imagined mixed readership, which spelling or word usage is most clear and most readable to the most number?