I've taken a few whitetails with a 20 and Brenneke slugs, taken many more with a 12 and slugs. I see no difference in effect or length of blood trails, IF the slugs are inserted properly.
Note, we have used some of the sabot loads in 12 ga for herd control. Blood trails run longer, and while we've managed to find them all, it took more effort and had a higher risk of loss, IMO. I'd stick to bore diameter slugs like the older Forster types and the various Brennekes. These do not shoot best in rifled bbls.
The Shotgun Forum has lots of old threads on this very subject, a little research will get you plenty of input....