Aren't Class 90's just upgraded 87's. So are there too many differences?
Power outputs are the same: 5,000hp continuous, 7,860hp one-hour.
(87101 was 4,850hp continous)
Top speed is 110 for both.
87s (except 87101) have tap-changer control, 90s (and 87101) have thyristors.
87s have the standard fully variable brake handle, 90s have the same six-step controller as in 91s.
90s also have speed set; E70 brake units enabling the transmitting of brake demands from such units in Mk3 and Mk4 DVTs.
Obviously they look different, have as mentioned different cab layouts and 87s weight slightly less (83.3 against 84.5 tonnes).
I believe the also 90s have a more modern rheostatic brake (forget the precise details of what this entails - possibly it doesn't cut out in neutral sections?), as well as an electronic wheelspin protection system rather than an anti-slip brake.
There are likely some more differences but those are the ones I can think of just now.