Viele, smaller blade, hollow ground, hole opener, liner lock, bolster, more belly.
Opportunist, longer blade, flat grind, stud (one side only), liner lock, wood, more wharncliffey, no clip
VG10 in the Viele, S30 V in the Opportunist.
Both are dressy. I like the longer blade of the Opportunist as well as the less belly. However, many like more belly on their blade than I do. I also prefer a flat grind to a hollow grind.
After all the talk of an eccentric stop pin, the Opportunist actually has one. Each facet of the pin is at a different distance from the center of the pin. You can adjust your liner lock for wear with it. I haven't tried that feature.
You probably have a better indexing and a bit of guard in the finger cutout for the Opportunist. In these knives, that shouldn't matter, but the argument is there to be made.
I own an Opportunist. I think it's prettier than the Viele and I use it for more formal occasions. It's not an EDC for me. The main thing I don't like about the Opportunist is the little pointy hump on the blade spine. It's just wrong for a blade that is otherwise so wharncliffe. It's sort of at the point a thumb ramp would be and is probably what the design is there for, but it breaks up the lines and isn't necessary to the blade IMHO.
I wouldn't want either as an EDC. I don't trust liner locks that much. But I explained why I like the Opportunist.
Phil