In my experience, it depends on the bullet and groove diameters. An under-size bullet will cause leading if cast too hard. An equivalent soft bullet will set-up on firing to fill the grooves and not cause the lead build up.
If the bullet diameter is groove diameter or very slightly above, you need to run a comparision test. You may not find any difference. Note that it is the groove diameter in the throat that counts.
Fred

What the barrel wants needs testing it will depend on the barrel its self and the propellent used. Earlier this year I saw some test targets that John Lewis published shooting 22:1 against 25:1 side by side. His results were better with the 22:1. It got me to thinking I had been using 30:1 for years because it shot well in 1 rifle. Since then I had been using 30:1 in all my guns. I made up a pot of 20:1 cast some bullets and went to the range. I shot the 2 sets of bullets side by side. First 5 of one then 5 of the other. In every case the 20:1 groups were noticeably smaller some as much as half the size of the 30:1. I also tried it with my .25 and got the same results. That convinced me that in my RKS barrels to go harder rather than softer. 
  I would caution against using antimony in your alloy. I believe the loss of ductility that comes with antimony is detrimental to accuracy. crush a bullet cast with lino and one cast with just tin-lead and look at the difference. The lino tends to crumble and the tin-lead to just deform.

40 Rod

SSShooter,

Bullet nose is also important when deciding between soft and hard alloys.  I had a heavy 38 caliber bullet mould made that gave me a very long, non-bore riding nose for use with black powder.  When I used my standard 20 to 1 lead/tin melt, accuracy was very poor.  When I switched to wheelweights and 2% tin, it became a tack driver.  The nose was slumping with the softer alloy.  Sometimes you have to go with what works.

feather

40 Rod don't hesitate to try 16-1 my 33-47 Bresien came into its own with this harder alloy although my 32-40 doesn't quite like em quite that hard. I have found the harder bullets will tend to shoot smaller and more consistent groups up to the point of the alloy being harder than a particular rifle will digest in regards to leading.

Joe bullet slump with the softer alloy spritzer bullets is a proven fact and the noses slump from 2 thous on up and never slump in the same direction. A very close friend of mine did extensive testing with nose slump by capturing and measuring fired bullets of different alloys and nose designs. And when you find the time to come back out this way or head to Montana, Raton or else where to compete in another match it is beyond me why you would feel you would be better served to save enough money for food, gas and accommodations to drive that many miles yet still have a hard time with buying and adding enough tin to win!

J.Louis

You lost me Joe one minute you are talking about harder bullets with 1 shot high and 5 lower in a dime size group all in the 25 ring so one would tend to think you were talking about bench rest shooting and then you say you have no interest in bench rest just off hand shooting using soft bullets at low velocity so now you have me totally confused.

Seems to me if the bullet nose slumps the fit is wrong for that rifle.  Might be able to fix it with harder alloy but the real problem is, bullet does not fit. 

Joe and I are in the same camp on Bench Rest shooting, Glad guys shoot bench so I can learn what the best loads are though. Keep at it tell us what works.

Boats

40 Rod tell me a little bit about your Borchardt bench rifle I always thought it would be the best choice for bench rest shooting and with Bernie doing the stock work I could only imagine how graceful it must be Bernie is a true artisan. The work he did on Dale's Frogmore Rifle is absolutely gorgeous.

By chance can you post a couple of pictures of it for us all to enjoy

J.Louis