First, I have 22/45s (5.5 Bull SS and 6 7/8 Govt. Comp Slabside), preferring them to the Mark II for several reasons that have been documented on pages previous. Tyr them against the MKII.
Second, I am not a fan of the tapered barrels. They get hot quick.
Find a place that you can shoot both that you are considering and make up your own mind. Determine if you are going to shoot iron sights or a red dot or scope...if you think a dot/scope is the way you want to go, be sure the gun you get is already drilled and tapped...you don't want to go to the trouble and expense of this if you can get a shooting iron that is ready to accept your mount/sight.
I shot both my 5.5 with iron sights and my 678 with a dot tonight and did well with both. They are very different, however, in balance and felt recoil (the open 5.5 jumps much more as there is less barrel weight than the 678 with the dot mounted on top).
Bottom line, your personal preference is what counts, so take the time to try things out.