I went through a similar -should I buy a second 870 - or just find a 28-30 inch barrel to use on my HD gun with 18.5" barrel.
I wanted to shoot clay targets.
I got lucky - found a 28" barrel on gun broker for $75 - I think at the time I figured if I could get a used barrel for under $100 I would take this route - much over $100 new gun.
I will say this -
I have shot hundreds and hundreds of EXTRA rounds through my HD 870 since I have the longer barrel.
No way I would go to the range and blast 100 rounds through my 18.5 inch barrel - but this is easy to do when shooting clays.
The benefit - the action on my 870 gets better and better with use.
If I was you I would still swap out the barrels between guns and put some extra rounds through your HD shotgun.