Here's my advice as someone who came in with a fixed-wing PPL.
It does help your packet. How much depends entirely on how many hours you have. The board apparently liked my 120 hours quite a bit, because it managed to cancel out a 2.9 GPA.
I would recommend getting the fixed wing over the rotary wing for the following reasons:
1. It's a hell of a lot cheaper, which is good considering that you'll get paid to get trained by the Army.
2. You'll come out of WOFT with a commercial and instrument rotary-wing rating (if you take the Military Equivalency course), so you're just adding to your skillset if you go fixed-wing prior to being selected.
3. The actual mechanical skills of flying a helicopter will come to you easily no matter which rating you get.
4. The knowledge that is really going to help once you get to primary (how to read a sectional, airspace, some aerodynamics, comfort with talking on the radio, etc.) will be taught to you in either fixed or rotary on the civilian side.
What I'm trying to say with all that is that there are a couple minor advantages to going fixed (namely, cost and widening your skill set). You might as well go fixed wing, because the skills that will actually help you while you're in flight school will be taught for both fixed and rotary on the civilian side.
(All that being said, helicopters are way more fun and I never want to go back to flying starch-wings again)