The two makes are quite similar and Paul Said, the owner of Mtarfa, is related to the Mdina owner. I think they are brothers.
Personally I don't think Mtarfa is inferior at all, but most of their items (as Le Casson eluded to elsewhere) are not very demanding or imaginative and tend to follow the same form. Early Mdina though, was originally started by Michael Harris (later of Isle of Wight glass - note the resemblance!) and his 'axe head' vases could be seen as contemporary of Whitefriars art glass - many, many excellent pieces that command high prices.
However, Mtarfa do make some very weird vases where a long neck is repeatedly folded over and I think they are fascinating! Then again, I'm a bit weird :lol: But these might be termed 'end of day' vases - Friday afternoon jobs where the glass makers let their hair down and go a bit wild.
If you want some images of these weirdies I'll gladly post them.
At the end of the day, if you like them, collect them! Most can be picked up remarkably cheaply and they may become tomorrow's collectables.
Best regards,
David