Well I've owned a number of 1990's cars, and am working on getting the remainder of 1994 cars I haven't owned.
As far as I know, Minichamps made the following models
Benetton 193B (Launch car) and the Benetton B194 - Onyx also made the B194 and a late season version with updated side pods which Minichamps didn't. Minichamps made various models of the Benetton throughout the year, as they were one of the sponsors.
Williams FW15C (Launch car) and the Williams FW16.
McLaren MP4/8 - very nice models
Onyx also made it.
Ferrari 412 T1 - early season versions made by Onyx and Minichamps. Mattel Elite have made a number of late season versions which are very nice, I have the Silverstone GP.
Jordan 194 - Minichamps have made it in their 10 year anniversary set (so you get it along with the 1993 car). Onyx made it on its own with its four drivers.
Tyrrell 022 - Both Minichamps and Onyx made a standard version of the car.
Sauber C13 - Minichamps have made an early season and mid season shape with both liveries. Onyx stook with the early season shape with various liveries - I've got the 200th GP De Cesaris on the way
, although I would have prefered the Minichamps version, but its pretty rare.
Simtek S941 - Minichamps have made it retrospectively, and the Ratzenberger model I have is superb, much better than their 1995 model.
Onyx have made the Ligier, Lotus and Pacific (using the Benetton shape).
If I remember correctly, there were no Footwork or Minardis made by Onyx or Minichamps, except expensive Tameo/BBR models. There is a cheap Corgi version of the Footwork, but not very accurate. I have seen a 1994 Footwork converted from the Onyx 1995 model which was nice, but I've only seen that around once.
Larrousse (kronenbourg & tourtel versions) is made by a French model maker Eligor - they are rare and expensive, and I am waiting for the right one to come at the right price.
Hope that helps.