Doesn't really bug me at all... in fact I don't really think you're even going out on a limb here.
I've got ADPs on them at #9 for Howard and #10 for MCab, so you're neck and neck anyway.
Current projections I'm looking at:
Trading forty points of average for 15 HR and 10 RBIs... nothing earth-shattering.
My biggest concern is the tiers at their positions... I feel a whole lot better about taking MCab at 8 or 9 and then having a long second tier at 1B to choose from (Berkman, Hafner, Tex, Morneau, Atkins, Lee, and Gonzo).
Doing it the other way (taking Howard at 8 or 9), your 3B tiers get a little shorter (Braun, ARam, Atkins... and maybe Chipper). The dropoff after this group into the third tier gets scary steep.
If you're definitely taking a 1B/3B combo in the 1st and 2nd round, I don't think you can make a bad move here, either way would be perfectly fine... but if you're leaving the door open for a good #1OF (like a Crawford or Sizemore) at the beginning of the second round, I'd much rather have MCab and then try to catch Hafner or Lee in the 3rd or 4th rounds. Otherwise you're taking Howard and an OF in the 1/2, and praying that nobody gets cute and takes ARam and Atkins before it gets back to you.