Here's the comparison photograph...
Top Row: MAC: From left to right; 'Viva Glam Gaga', 'Lady Danger', 'Midimauve'
Bottom Row: MUA: From left to right; 'Shade 4', 'Shade 13', 'Shade 11'
I honestly couldn't believe that there was such little difference between the two brands. The are both exceptional brands and I would recommend buying lipsticks from any of them but if there's no difference between the two then you may as well go for the cheaper option.
The only thing I will say is that MUA do not offer a wide range of lipsticks unlike MAC which is a shame.
Hope you like this dupe post! The dupe for 'Midimauve' looks better in the flesh than it does on camera too!