Infantry Officer,
Let me share some of my experiences with you. First, thanks for expressing interest in serving. Would love to have you aboard. I can speak mostly to the Army, but have worked a bit with the Devil Dogs. Having had a few friends go through OCS the last year or so, you should have little trouble getting branch Infantry. Keep in mind some of the best of our organization are Infantry officers, so don't let others dissuade you.
As for the 18x program, I'm not a big fan. I see a lot of 18x behind their peers through no fault of their own, only because the most appropriate Special Forces soldier has a few years in the Army and is more mature/older. See the website Professionalsoldiers.com for a lot of good information.
I reckon you will be bored with the conventional army and should look into the SOF community in any branch of service. As an Infantry officer in the Army, you get the opportunity to go to SF selection as a Captain after your first assignment. This might be appropriate for you. Otherwise, I would say the Marine Corps offers a better opportunity if you want to deal with people who are a bit more motivated in regular unit assignments.
I don't think I've heard of an Infantry officer that hasn't had the opportunity to go straight to Ranger school before they go to their first unit. The Rangers and Special Forces represent the opposite ends of the spectrum with regard to attitude. The Rangers tend to be boisterous and energy-filled. SF are laid-back and Quiet Professionals. Neither is better than the other, it's just what suits your bent. (There is quite a different mission-set between the two as well.)
One last thing, secure an Airborne slot when you sign your contract. You will already be at Benning for OCS and can move into jump school rather easily. Otherwise you will have to go to your first unit and then fight for an Airborne slot. (That is if you choose the Army of course.)