Holt had 145 targets to 85 for Holmes. Assuming both stay healthy, Holt's targets are definitely not going down this year and Holmes would need to see his increase significantly because I don't think he can maintain the lofty, league leading 18.1 ypc from last season.
And that is exactly the reason in a non-PPR league I'd go with Holmes over Holt.
One thing to add is the 85 targets is not a fair measuring stick since Holmes missed 3 games. Over a full 16 game season Holmes numbers would have projected to 104.6 targets.
Holt edged Holmes in total fantasy points but Holmes actually beat him in fantasy points per game in non-PPR leagues. So the way I see it is if Holmes can outproduce Holt with less targets and I feel he has far greater potential to increase his targets than Holt has chance to increase his targets than Holmes would be a no brainer pick for me. Again in non-PPR league.
Regarding the issue of his YPC. He averaged 16.8 on a similar amount of catches his rookie year so he has shown a propensity for a high YPC. Now some of that is why he also is not targeted as frequently is because he runs a lot of deep routes. This is were I say something has to give one way or the other for Holmes and either way he'll come out as a good producer. If the Steelers continue to feature Holmes on the same type of patterns he has always run he might not see a sizable spike in targets but his YPC would probably remain high. If the Steelers do as I think they will do, which is feature Holmes in an expanded role his targets will increase but he YPC will decrease but that will be more than offset by his increased targets.