Main differences that I can see is that the Tamiya has a servo on the rh side, where the Revell one has the heater air ducting. That may well be correct, due to them representing different versions, but my knowledge on specifics like that about Minis is rather limited. The Revell also has a larger airbox, whereas the Tamiya has individual filters., which will also make the engine bays look different. The Revell also includes the crankcase breather system, which is missing from the Tamiya, and of course plug leads.
However, I haven't seen the Revell kit in person, just photos. Geoff - what's your view on the above?
The Tamiya version is the 1275c.c Cooper S and that did have a servo fitted to the brake system, but it also had a fresh air ducting to the heater, which is missing from the Tamiya kit, also the hydraulic clutch master cylinder is also missing from the kit.
Cooper S 1275 engine bay
Cooper 998 Engine bay
The Revell version is the 998 c.c Cooper, which the real thing, although still had the 7" discs fitted to the front didn't have a servo fitted to the brake system, so the Revell kit is correct, also, as you have mentioned it does have the fresh air ducting for the heater, it also has the voltage regulator fitted to the inner wing and the clutch master cylinder, and the crackcase breather, inlet manifold and air cleaner is also better than the Tamiya offering, so the Revell has to get my vote for the more detailed and more prototypical kit ....