The CPU board is literally the same. (Although there are variants/revisions.)
The video board is literally the same. (Although there are variants/revisions.)
The CRT, flyback, power switch/keyboard jack are literally the same.
The keyboard is literally the same.
The mouse is internally the same, although: an early Lisa mouse may have the squeeze clips instead of jackscrews, and an early 2/5 motherboard may have the clip retainer that interferes with plugging in a regular DE-9 plug mouse cable.
The chassis is the same, although some units have the vertical cutout between the drive cage and the card cage which can be useful for passing cables but is otherwise unused.
The PSU's are compatible, although the 2/10 "DataPower" variety is preferred as providing more power and generally considered a better unit. The extra power is really needed if you have an internal hard drive (the mechanical kind).
The discrete wiring harness from the PSU is slightly different; the 2/10 variety has the 8 pin widget power connector, and the 2/5 variety has an unused 3 pin connector with 2 wires. Unless you have a widget or want to use it's power connector, you probably don't care which you have.
The ribbon cable harness is different; the 2/10 has a 26 pin and a 20 pin connector, the 2/5 has two 26 pin connectors. These should be matched to the motherboard, although there is this topic about them including making a universal harness (probably only useful when your card cage is not in the chassis):
Chassis Ribbon Cable. They are mounted slightly differently in the chassis, resulting in the different positions of the motherboard card edge connectors.
If you disconnect the 26/20 pin ribbon cables in the drive cage, you can use either I/O board. The software will think it is a 2/5 or 2/10 depending on which one you use.
The 3.5" floppy drives are the same. Assuming you have a matching set of motherboard, I/O Board and ribbon cable harness: In a 2/5 you need the Lite Adapter card, in the 2/10 you don't.
What did I miss?
edit:
The Memory boards are literally the same.
The frame that holds the cards in the card cage is literally the same, however, the metal pan that the frame and the motherboard are attached to is different. The pan includes the rear connector panel for the motherboard. The 2/10 version has a back panel interrupt button and two DB-25 connectors (for the serial ports). The 2/5 version has three DB-25 connectors (the third being the built-in parallel port), and does not have an interrupt button.
The motherboards are different along the lines of the pan described above. The 2/10 parallel port is routed to the internal ribbon cable instead of a back panel DB-25. The Sapient variant of the 2/5 motherboard has an interrupt button where the RCA video out normally is (the RCA jack is still there but on the inside).
What else did I miss?