SR4756 is the slower of the two, thus will allow higher velocities. Blue Dot is also a prime powder for these sorts of loads.

The Lyman shotshell loading manuals have excellent data near the back for slug and round ball loads, 12 or 20 gauge.

SR4756 and Blue Dot are usually close, with Blue Dot generally only a few fps faster. 4756 strikes me as being more forgiving with lighter loads - easier ignition.

12- Fed Gold Medal Cases, 4756 will break 1,600fps with 475gr. Lyman cast Fosters. With their 525gr. "Pellet" slug, 1,500fps is it- faster with the even slower powders.

In 20 bore, Blue Dot is ahead by more, however 4756 does a good job, even if it is 100fps+ slower most of the time.
For example, 350gr. Lyman sabot slug, 4756 at 1,436fps and Blue Dot at 1,596fps using Fed paper base wad, but with Fiocchi all plastic cases, 4756 makes 1,623 with the 350gr. and Blue Dot isn't listed.