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An EL-90 is an open design with open space between the speakers and the pedalboard
An EL-90T is a cabinet housing
An EL-90I is white (Ivory)

An EL-90 can easily be disassembled into 5 pieces:

• the upper part (2 screws under the keyboards, just in front of the speakers and 2 screws in the back between the upper part and the speaker part)
• the speakers and amplification part (4 screws on the top and 4 screws behind plastic caps on the back)
• two side panels (two screws each, on top of the pedalboard, under a plastic cap)
• pedal board

The visible differences between an EL-90 and an EL-70 are:

• EL-70 has 8 & EL-90 has 16 presets between the keyboards
• EL-70 has 1 & EL-90 has 2 pedal voices
• EL-90 has an expression pedal
• EL-90 has a bigger screen (same resolution!)

• EL-70 has 79 & EL-90 has 131 voices (for instance panflute)
• EL-70 has 4 & EL-90 has 6 speakers
• EL-90 has better sounding voices
• EL-90 has initial & after touch on the pedals
• More software is available for EL-90
• and some more

To put an EL-90 (probably the whole EL-series) in SELF CHECK MODE:
Hold down the C# Chord on the lower keyboard, starting with the second C# - while switching on the organ.
(This also displays the software versions)

Software versions I have ‘seen’ so far are:
EL-70: (C)'91/03/13 V.4.2(MAIN) V.5.0(MDRI)
EL-90: (C)'91/05/29 V.5.1(MAIN) V.9.0(MDRI)
EL-90: (C)'91/09/05 V 5.3(MAIN) V.E.0(MDRI)

I have no idea what the differences between these versions are.
Unfortunately software of the EL-x0 series can only be updated by replacing ROM's.