TD64 is probably the easiest to support from a coders point of view (i.e. a simple extension of TD32) but since NSD64 became the official OS 3.5/3.9 standard I think it should always be supported. There is simply no point in continuing the debate about which is better now.
Also, support for TD64 is optional since NSDpatch can patch TD64 to emulate NSD64.
