I've shot both extensively and they are both extremely accurate, however the H&N heavy Slugs and the HP II line are just a bit more accurate, at least in my guns.
I have shot several of both plus several of the competing offerings in my .25 King. The Heavy Slugs in 44 grain are the most accurate so far with the ZAN 37 Gr and Alteros 50 Grain right behind. The Javelins were about middle of the pack.