I mean, which is better overall for variation/strength building out of resistance training or free weights.
Define “better”. There’s a big difference between developing “beach” muscles, and developing functional strength you can actually use. It’s not simply a case of saying machines vs free weights. I mix up both though my machine workouts tend to focus on core stuff rather than guns, pecs etc. My free weight stuff revolves around deadlifts, squats, farmers walks etc.
And doing a small 30 min fast cycle going to build up cardio strength and generally improve my fitness overall
That’ll get you fit for a 30 minute fast cycle. If that’s all you ever do on bike then it’ll be great, otherwise not so great.