I think Prestigio is mainly an importer and distributor of low-cost Chinese tech products, and puts their own branding on them. Whereas Toshiba is the actual manufacturer of their own products. Myself, I'd certainly get the Toshiba. I don't even know where Prestigio are based, contact is anonymous webform/email only, and the website is registered in Cyprus. They're probably OK though for warranty and that, because they appear to sell through reputable retailers, like Littlewoods in the UK.
If you got the money to spend on the Toshiba AT300, you've got a number of options for good devices from several reputable manufacturers, like Samsung, Asus, LG, Motorola, HTC, Sony, etc. Myself I'd go for the Asus Transformer Prime, but is probably a little bit more expensive.