1885 High Wall or 1874 Sharps?

[Ed333]

I have been wanting a 45-70 for some time now, one heavy enough to let me do some serious target shooting (which ruled out a couple of nice Ruger #1s I have seen). Right now there are two lightly used Ubertis available at two different LGS, an 1885 Highwall, straight grip, decent trigger, crescent buttplate, octagonal 30" barrel, buckhorn open sights. The other (only slightly more expensive) is an 1874 Sharps, crescent butt plate, straight grip, double trigger, ladder sight, maybe a tad heavier than the Highwall, but not enough to make the difference, round barrel (which I have not seen in the Uberti or Pedersoli lineups, they all seem to be octagonal on current offerings.) Neither one is one of the fancy models.
Is there a reason to prefer one over the other? Will the big external hammer on the Sharps mean slower lock time and a distraction? I will be shooting from a rest and rear bag, start on a bench at 100 yards, work up to 300 prone, maybe 600 at some point down the road if the bug really takes. I am a current F class shooter with a 6mmBR Norma, so this will be something different. If the bug takes, I will probably install a good rear tang vernier sight and front tunnel/globe on either one, so that is a toss-up. I assume the existing sights on the Sharps are slightly better, and more useful at longer ranges. Accuracy is important to me...which way should I go?
PS- I am aware that a shotgun butt is preferable, but figure if it beats me up too badly, I have some slip-on wrap-arounds I can try, I am tall and can usually use a little more length of pull.