I don't think you can go wrong with a 223 barrel. It's so cheap to shoot, it covers the plinking aspect very well. And having it in a single shot makes you really savor every shot
If they still have the 270 UltraComps in, I say definately pick one up while they are availible, give them a call to see if they are though. I personally don't plan on using my Comp anymore for hunting applications, due to it being so loud. But I am making a Target shooter out of mine. These barrels are a true joy to shoot, and recoil is non existant IMHO.
The 44Mag barrels are a ton of fun, but have had a track record for being a little fussy. I think the 357 would be a great option, being able to shoot 38's in it is a cool plus and from all reports are very quit to shoot.
Tons of great options. And as for the 45-70, If you reload it's a great choice. These have had a good reputation of being shooters too.
It's cool being able to build your own Handi Library of calibers, and do it all of 1 frame.
-Aaron