This is a case where it's as simple as the value of the position, and strength of the class. Also, keep in mind that I don't draft vertically. For example, I won't necessarily draft the player that's #32 over the player that's #33 in every instance. Talent wise, you can obviously see that I view Hopkins and Stacy extremely close together. I believe Hopkins to be as quality of a receiver as Stacy is a running back, and vice versa.
However, vertical grade is also determined by the competition that each prospect is competing against at their respective positions. The WR class that Hopkins is competing with is deeper and stronger than the RB class that Stacy is competing against. Stacy is my #3 RB, whereas Hopkins is my #9 WR. They happen to fall into the same range on my vertical big board because the strength of the WR class is stronger than the RB class.
I wouldn't pass up my #3 rated player at any position for my #9 rated player at another position if the need there is equal for both. You've often heard me talk about how I wouldn't pass up the #1 safety on my board (Eric Reid) at the top of the 2nd round for the #4 or #5 cornerbacks on my board at the top of the 2nd round, even though they all fall grade wise into a range at the top of the 2nd round. This is where the term "value" has a purpose, and why teams don't draft vertically.
Hopkins ranks one spot higher on my big board because I tend to give underclassman the advantage when stacking equal talents vertically. Again, that doesn't mean I'd actually draft that way. This is why I think it's important to have position rankings to refer back to in addition to your big board. It's how you guage value, and make the best decision in terms of selecting one over the other.
I think it's a good idea to draft vertically in the top 10 picks or so because that's where the elite talent is, and elite talent always takes precedence over need. But once you get beyond those 7 or 8 elite talents, I think it's a good idea to start considering value at that point. If I'm choosing between the #1 TE, #6 WR, and #3 CB on my board, I'm going with the #1 TE because that's where I'm getting the best value.