Considering the fact they're both made by the same company, I'd likely choose the Ridgid set for the warranty and larger batteries just as you said.
I'd disagree with any need for the Milwaukee fuel tools for your stated use of auto shop work. The fuel drill, AFAIK, is only available in a hammer drill version. Doubt you'd encounter much concrete in a car. The fuel impact driver has marginally more max torque but the bare tool without batteries or charger costs as much as either of the 2 tool kits you're looking at now.
If you were a carpenter or other construction worker, the fuel kit would make sense. Bigger batteries for hogging holes thru framing lumber or driving deck screws for half the day, or drilling for concrete anchors, but those aren't auto tech types of work.
I've had the Ryobi drill and driver kit for about 4 years. Also made by the same company (Techtronic) and with roughly the same power levels as the kits you're looking at. As examples of actual use, I've tapped 300 10-32 holes thru 1/4" steel plate with the drill on a single charge. I've used the driver to replace the front clip on my pickup on basically every fastener other than the bumper and it's braces. Did all the disassembly one day, put the battery on the charger overnight, and all the assembly the next day without needing to charge or swap batteries. So far, I've not seen a need for bigger batteries or more power, and I doubt you would either for your intended use.
If I was buying again, I'd still buy the Ryobi kit due to the wide selection of other reasonably priced tools using the same battery platform. Shortly after I bought the original kit, I needed to drill out a broken bolt on the front of an engine. Either remove the radiator and a/c condenser to get access for the standard drill, or buy a right angle drill. The bare right angle drill from Ryobi was $79, so I bought it. Since then, it seems like almost every job finds me using the RA drill to get to one or more spots where the standard drill won't fit. I don't think Ridgid has a RA drill in their lineup. Milwaukee has one, but it's a $300 monster made for running big forstner drills thru framing lumber.