A wise choice. Also, you will find that line officers rank faster while staff officers generally have to fight to get good evals. Quality of life is generally the difference to my understanding. Staff officers have a good quality of life and enjoy good stations while line officers sometimes take crappy orders and deal with some rather stupid stuff. Both have their ups and downs, the real question is what do you want? Do you want to see patients and get lots of great clinical experiences that some civilians will never get, or do you want the chance to command forces and develop attack plans? Two very different routes.
Either way I think that any officer path is rewarding.
From my 8 years in the Army I would say that whether you are a line officer or a staff officer doesn't really make any difference. There is definitely a difference of perspective and priorities, however. The best way I have had this explained in comparison is the "clean boot vs dirty boot" Soldiers/Officers. Some thrive in austere field environments/deployments where they operate at the tactical level. Others thrive in the planning/management area and are generally your go-to Soldiers while in garrison. The aim is to have a balance of these two, however everyone is different with varying degrees of experiences and values and styles of leadership. In terms of one ranking up faster or getting better assignments, I've never seen one get preference over the other. It's really based on needs of the Army, and your performance and potential. BUT (big BUT) like most things in life who you get to know can pay off a lot, and your Staff officers generally have a much better chance of networking with well-placed people.
To the OP: In terms of PA in the Army, like most Officer billets, you will see a mix of both line and staff work. As a new Army PA, you will most likely be put in a line Battalion (Infantry, Armor, Artillery, Cavalary, etc.) and you will spend your time dealing with acute/chronic complains and Soldiers just trying to get out of PT during sick call. Additionally, you'll have Army Medics working with you, who you will be responsible for training and mentoring. When I was a Medic, our PA would let us do all kinds of procedures and was a big fan of learning by getting our hands dirty. The admin side as handled by the Medical Services Officer (the Medical Platoon Leader) but the actually "doing" i.e. treating patients, patching up wounds, administering meds, etc. was handled by the Enlisted Medics and the PA. Later on, as a Senior PA (probably CPT and above) you might find yourself in a more clinical situation.
I would agree with though, you really need to be sure if it's something you REALLY want to do. I know a few people who applied to IPAP and didn't get accepted the first or even second times. I have an old AIT buddy who applied twice, got denied twice, and not he's a Flight Medic and loves it. I would recommend you pull the trigger on your OCS packet, do some time as an Officer and then see if the medical field is something you really want to do.