I've used both, and also couldn't tell the difference; however the official figures are 3200ml for Delta Form, Medium X-Plus and 3600ml for the same variant of Abriform, so around a 12% gap [assuming my maths is, correct]. The Delta Form are somewhat more cheaper though, at least here in the UK (around 20%) so do make more sense in some aspects.
Personally, I'm not too concerned what the colour of the product is, providing it isn't something that would really make me uncomfortable, such as pink!