Asus also said the Prime was an "enthusiast"device, so never intended for mass sales. Most major retailers do not stock it.
Points about the 300:
$100 less.
Faster ram.
Thinner and lighter than the Transformer 1.
Based on feedback at XDA, much better wifi than the Prime, so the plastic panel is a good thing.
Though not IPS+, still an IPS panel.
Will be much easier to purchase, since will actually be stocked.
Has the same chipset, but slightly underclocked.
IMO, the better wifi, lower price and faster ram is key. This tablet should sell well.
Added:
My only complaint is the colors and that is trivial.