What are you using when you are doing any kind of training?

Are you mostly just using birdshot for arms manipulation and recoil management? Or are you using cheap buckshot/slugs?

Both.  

Birdshot that functions well in the chosen shotgun, and buckshot that does the same for the reasons that you state.

You can shoot a couple hundred rounds of birdshot in a day without discomfort if your gun fits.  Buckshot, not so much, so the buckshot use is intermittent to help "remember" what it is like.

The ratio of bird to buck is about 5 to 1 roughly in my training. You need to fire full power buck to see where it patterns, recoil management etc.....but most of your training is done with birdshot. It's also much cheaper that way of course.

I like to use Suprema 00 Buck for manipulations and recoil control in my Beretta 1301T, it runs about 10 bucks for a box of 25. I like having a more realistic recoil, seems more sensible to me. It does pattern worse than throwing a handful of marbles at paper though.

Depends on what I'm practicing.  If its strictly manipulation I'll use birdshot.  For accuracy I'll use slugs.

The cheapest well functioning birdshot that can be found for SG training.  Example 25 rds for $4 - 5.  For use on Steel plates, clays, etc.
A bunch of cheap buckshot  ( Ex. 5  rds for $3)would be next for training in limited amounts due to cost.
Finally, shoot some good quality / Real World Use Buck or Slug for confirmation of patterns, usually a "Tactical or Low recoil" load as made by Federal, Remington, or Winchester.

SG involves lots of hands on Gun training, Reloading all the time, lots of Reps.  So Cheap Birdshot is the key.

i use both.  slugs will give more accuracy

This is my approach as well.

Most of my shotgun ammo is a hodgepodge of birdshot accumulated from a local pawnshop. People will bring in their old scattergun and two-three boxes of shells to sell. The store can't re sell the ammo, so it ends up in the back room...from there, it ends up in my inventory.....The vast majority of this ammo is "dove loads"...$4-5 a box ammo, some of it 30-40 years old- doesn't matter. I've shot a shit ton of paper hulled dove loads from the 60's and 70's in just the last year, not a problem. The last bunch was some Canadian made Imperials and some JC Higgins store brand shells- probably made by Federal, Winchester etc...,Who Knows.......

I shoot a pretty good variety of inexpensive buckshot- Sellier, Rio, Fiocchi etc.....

Then I do my "precision shooting" with Federal Flight Control.

For buckshot I reload .311/.375 round balls I cast.  My 870 eats them all day and patterns them nicely.  Shoot lots of cheap dove loads also.

CD