Did 2 years spin classes on a Stages SC3 indoor spin bike at the gym. About a year later got a Kickr 2017, used Erg for 2 years. Then last two years I switched to mostly outside, and inside Kickr in slope/standard/level mode.
Indoors I find everything easier in resistance (SC3) or slope/standard/level mode (Kickr).
Gym rides before getting the Kickr, have a few structured rides in the gym including these 40/20s comparing gym to Kickr Erg/PowerMatch (Taylor),
and this Sept 2017 gym ride
of ~1.75 hours on the Stages gym bike and it starts with 5-min vo2 intervals before the class started, with the last 50 minutes being a spin class. Zwift running/recording on my iPhone, and those are in pure resistance mode (no erg on that gym bike). A couple months later I bought the Kickr v3 direct-drive and was scratching my head about why everyone was saying Erg was a game changer.