1/ the old way - $OUTPUT->single_button($url, $label, ...) which allows single_button object in first param too

benefits:

• extremely easy migration from print_single_button() in 1.9
• easy for new ppl that use code completion in IDEs - type $OUUTPUT ==> then you get list of methods ==> then you get all single_button() parameters and links to advanced usage
• make methods are optionally possible too

2/ make factory methods

$OUTPUT->print(single_button::make($url, $label, ...))

problems:

• no code completion possible
• totally different from 1.9
• custom OOP style (not in Java or anything else I know)
• Eloy does not like static methods :-P

3/ $OUTPUT->button(html_form::make_single_button())
it has the worst of both worlds

Please submit more cons and pros as comments