Rambo is their top banana; Rhino is middling and Amigo is the budget range. Think of it a bit like M&S/Waitrose; Tesco/Sainsbury's/Asda/Morrisons; Lidl/Netto/Aldi/Farmfoods/Iceland!
I have loads of Rambo and a few Rhino. I've never bought Amigo. I think the difference has to be the denier.
I have a Rambo and Amigo - HATE my Rambo (Duo as well so damn expensive!) for many reasons but love my Amigo (just bought 2 more). So dont always think pricey is better
I have rambos, rhinos and amigos, I found that the quality tends to be better in the rambo, then rhino etc, I have never had to replace a Rambo (as yet) but the amigos i have, they usually last approx 2 years then the clips and buckles break and tears start to appear.
If your on a budget i would def reccomend amigos, but be aware they wont last forever (for the price you dont mind) I have never had a rhino turnout only stable rugs and they have been excellent but i do love my Rambo turnouts, I have heavy weight wug plus's for the winter and they keep the horses toasty and dry, then i have lighter weight amigo turnouts for the autumn / spring etc, it works well for me.