A reissue might be cheaper/easier to obtain (and in better condition) if you don't mind reissues. The reissues also come with more accessories and can actually shoot bullets (the original G1 toy just kind spits them out as the launcher spring is heavily weakened for safety reasons -- not that the original even comes w/ bullets). Although mouldwise I prefer the original ... the reissue has a slight variation to the mould design of one shoulder which most people probably don't notice or care about, but I prefer the design on the original. But I don't know if it's worth paying inflated aftermarket prices for a shoulder (and honestly, the modified shoulder on the reissue is perfectly fine too ).

Also remember that NSW law requires collectors to have a permit to own replica gun Transformer toys like G1 or MP Megatron (if you don't have already ). Last thing you want is to order a Megatron and then have Customs seize/destroy it.
http://megatron.net.au/