Regarding the American English vs British English debate and which one is best for students to learn, the standards of ESL text books as they apply to Thai schools and businesses are of such an appallingly low level of quality and effectiveness that the difference in what side of the pond it hails from, is almost a non issue. The people who come up with these nuisance books have never been outside their own country and have no idea what they are doing.
The general distinction between the two (in my experience) is that British based books are harder to learn from. The British books make too many assumptions about what students already know. They focus more on grammar and the books are just too 'busy' and dull.
The Americans make it easier (more white space/pictures, etc) but they still use the word 'soccer' so they obviously haven't tippee-toed into the real world of TEFL either. NOBODY outside North America uses the word 'soccer.' And nobody in Asia is called Maria Sanchez or Pedro Escobar!
I've seen some series' of books start out with good intentions and sound ideas but veer off the straight and narrow after a couple of levels.
All the books used in Thailand are crap, but the most neglected and barren area of GOOD materials is for teenagers. Writers and educators who come up with these God-awful books just aren't even on the same planet as their target audience. I end up re-writing all the books for whoever hires me or writing materials from scratch. It takes time but the benefits of doing this are light years ahead of anything published by the anonymous dolts at Macmillan or Cambridge, etc.
So, American English or British English? Just give me plain, easily digestible English, that people actually use, please!