Anyone know the difference between a 105 & an ultegra 42 tooth middle chain ring? I'm mostly curious about performance, but also the weight difference. I checked a few sites and couldn't find the respective weights.
Spec' Tarmac (road), Ridley Fenix (commuter & fast tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder)
Definitely some 105 rings are stamped and some Ultegra rings are machined. Machined rings cost more to manufacture and last longer. It's hard to know whether this applies to the two rings that you are considering.
Hardness affects durability. Machining does not significantly alter the hardness of Al, therefore durability should not increase. Machining can yield much tigher tolerances. However, the steel chain will quickly wear away any minor burrs during stamping.
High-end components rarely improve the joy of cycling. There will be negligible speed improvement for 99.99% of the cyclists. It's all about the MOTOR.