The circuit designs used in the ISA range are essentially the same as those in the Red range so if you are looking for the classic Focusrite Red sound then the ISA range will deliver that to you, plus more. The Red range was developed some years ago, before home recording and computer-based DAW set ups were so popular, and in terms of features they offer a lot less than the current ISA models. D.I. instrument inputs, variable mic pre impedance, insert points and also the optional high quality ADC cards are some of the things that are attracting people away from the Red range and towards the newer ISA products. Also the lower price point of the current ISA units cannot go unmentioned!
Subtle differences in sonic character do arise between the two ranges owing to PCB layout and also some units having transformer balanced outputs whilst others use electronic balancing. These differences are minimal however and when choosing which unit to buy it is best to weigh up the features each unit offers and decide on the one that will best suit your needs.