Treadmill or bike trainer?

I was thinking of getting a bicycle trainer for myself for Christmas and using with my son's bike (which he has outgrown but is fine for me). And then my friend told me she's selling her Nordic Track treadmill. The prices of the two are comparable. The setup footprint is comparable, although the treadmill takes up more square footage. But I've got the space for either if I rearrange the room.

I'm leaning toward the treadmill for some reason, possibly because I haven't been on a road bike in a looooong time (stationary bike at the gym, yes) and have been doing mostly walking or walk/jogging for exercise.

Anyone have an opinion on one vs. the other? Anecdotes? I'm aware of the potential dust-collector factor, which is also a consideration...although I've been thinking about this for a long time and would really like a home-based option for when the weather is bad/hot/cold etc.

Thanks!

Really just going to come down to which one you will use.

Do you have a preference on which activity you like?

For me I prefer cycling over running, plus I use the bike for actual riding when the weather is good.

I also find the difference between running on the treadmill and running on the street to be a bigger difference.

I can run for much longer on the treadmill than I can out in the world, so I don't feel like it is a good training tool for me.  It unfortunately is a necessary tool.

I used to believe that I got a better workout from running, but lately I have been doing some real high intensity spin classes and training sessions and while I don't get out of breath as much as I do when I'm running, I feel like I have really worked my legs and my workout clothes are drenched with sweat when I finish.