You'll see more accurate projections with an 18 than you will with a 10. So the "stud" you see with a 10 may not be quite so attractive as he would be with an 18. So unless you're skilled enough to rate prospect solely by current ratings, age and makeup, you'll want more accurate projections.
Also, you get more prospects to choose from with an 18 as opposed to 10.
Overall, you get far more value from the draft than you will for IFA. First-round draftees are generally much cheaper to sign than will a comparable IFA who's the subject of a bidding war.
If you're ONLY looking for studs, you'll probably have better luck dropping $30m on the top IFA's. But most likely you're going to want and need more depth on your roster than just a handful of stud IFA's. That's where the value of the 2nd-5th round of the draft comes in. If you go 0/0 with your draft budget, you're basically getting (maybe) a decent first rounder and nothing else.