I have a Lipault 50cm 2 wheeler and a Salsa Air IATA cabin trolley. I've used both as my main luggage for trips of longer than one week. Here are some observations (in no particular order):
1. Build quality - Rimowa wins hands down. Wheels especially are of significantly better quality. The nylon used by Lipault is not as durable as, say, that used by Tom Bihn, but if you are only carrying on it's fine. I took my Lipault as my main luggage on a recent trip to Bhutan and it coped without any issues.
2. Capacity - the Lipault holds more. Mine has three main compartments (plus a further external pocket) - two clamshell style and one large flat outer compartment (with a removable suiter) that unzips on three sides. But when it's fully packed, it can be difficult to fit it into the overhead compartment of some planes, because the depth measurement exceeds 9" pretty easily (a function of the give in the nylon and lack of structure of the case).
3. Structure - obviously the Rimowa delivers more structure! The Lipault is unstructured even by soft luggage standards. There's no frame of any kind so the only sold structure is the handle mechanism. I use packing cubes in all my luggage and this is particularly helpful when packing the Lipault.
4. Weight - I don't have the details to hand but from memory the Lipault is lighter - critical for European and Asian airlines. But that said, I've never had any trouble with airline carry-on weight limits when using the Rimowa (including in Asia and Australia, where a 7kg limit s pretty standard).
5. Ease of use. When the Lipault is fully packed, its lack of structure tends to result in me bearing more of the weight of the bag through my arm and shoulder because there is no structure in the bag to support the weight except that provided by the handle mechanism itself. The structured Rimowa doesn't have this problem even when it's being used as a 2 wheeler.
6. I tend to choose the Rimowa for city travel but I feel it looks too "urban professional" for some destinations (eg country, more remote places), where a less structured bag seems somehow more appropriate (just my opinion!)
7. Investment - for the money Rimowa delivers more IMO.
HTH.