I would have with a 4x, like the Leupold. I had one on a .45/70 Marlin and it worked very well, with ample FOV and ample magnification for the sorts of distances a hot-loaded .45/70 (and by extension, a .444) can reach. I also have a 4x on my 9.3x74 double rifle, and it works a treat there, for similar distances. The 4x Leupold also is very light and, importantly, has a small diameter eyepiece compared to some (especially many variables), so you can mount it low and still access the hammer easily.
I have owned a number of 6x scopes, and still have three of them. IMHO while they are a very versatile choice, they do give away a bit of FOV, and hence speed on close-in fast-moving critters. Not my choice for a .444 for that reason.
YMMV.
