The short answer IMO is if your deck has enough grey viable toss ammo that running one of them is correct, you should have both.
Longer answer, in a toss battle initiative matters less because you are planning to 1 shot them (If you can't 1 shot the Hero you shouldn't be blocking with a Spearman). 6/6 is already high enough you will lose initiative a decent amount of the time, especially since you should be using Spearmen to block small Heroes that you can 1 shot. The only danger is being 1 shot yourself, and 7/7 stats means all the 6/* GEs no longer make you lose and this is quite valuable. Also having a rainbow evil character in territory can be a liability. In the situation that all your enhancements are grey and you must run exact one of these two characters, I would pick Roman Spearmen.
