Dear Malc,
Whilst in all fairness I have not dealt with the other suppliers you mention, I think the main possible advantage is that modelflags.com only deal in flags and nothing else, thus they are a totally dedicated service. From experience, nothing is any trouble, and every effort is made to get you exactly what is needed - and if one makes the appropriate nice noises one off's can be done (if an ill. is available of course). The bulk of their business is to the US it seems, as not that many people seem to have heard about them here - or know how good their service is anyway. No, I am not connected with it!!! - it's just that if a supplier (indeed any supplier), gives me an A1 100% service, I will support them.
Regards, Bernard2