What’s the difference between chains and bands?

I’m running bench press specialization now and found that there are chains and band workout in .
I haven’t bought chains yet so I used bands but the RPE was so high .Maybe using band is harder than using chains?
Or maybe the band I use is too heavy ? I’m really confused.
And Idk how heavy bands&chains should I choose for the workout.should I add chains’ weight to total weight or not?
If you could help me, I'd really appreciate it.

And another question is that when I do decline bench press i feel very uncomfortable ,even just lying on the decline bench. So I change it to flat bench press and done it decently. Is this gonna work?