I was going to buy both - I have my Amiga club membership and am entitled to $50 from OS4.
However, having used MorphOS for a while, I really don't think OS4 could offer me anything extra, at least not £500+ worth that I would need to spend to run it (plus all the other components needed to make a working system). So, unless OS4 is available for the Pegasos, I will donate my $50 coupon to someone who really wants OS4.
