Hi Andy,
Our team's initial considerations were 15.5, 15.7 SP132 or 16.0.
My preference was 16.0 since we'll have to take that path at some point anyway.
There was some resistance to this since it's a bigger testing effort. But our position was that going to 15.7 (or 15.5) was really only a stopping-point on the way. So the perceived saving in effort was a false economy.
The "bigger testing effort" bugbear was a fight I was willing to take on!!!
And then I learnt that there is no downgrade path from 16.0 to any 15.x version. No dump and load. No in-place downgrade. Unless you have a repserver license (I am assuming you can go back using repserver) you can't go back. No. You have to suck it and get through somehow or accept data loss and work through your process-based data recovery.
And accepting data loss isn't an option in our environment.
At the end of our 12.5 -> 15.0 migration the knives came out and our organisation with seething with anger at the inability to downgrade and the problems we had encountered. The CEO was talking publically about spending a few million to migrate to Oracle DBMS and we wore some major reputation damage with our customers and Board. This is damage form which we still haven't fully recovered.
Sybase (yes they were still Sybase then) gave us their solemn oath that all future major-version upgrade paths would come with an officially supported downgrade path.
Maybe that got lost when Sybase became SAP. But it is the major hurdle in a 16.0 upgrade at this point in time.