Rowing or paddling: which is lower impact?

Without starting a religious debate, which is lower impact: rowing or paddling?

I love kayaking and currently paddle a surf ski in flat and small conditions for fun and fitness. I recently injured my shoulder and I am doing physio therapy to strengthen the rotator cuff. Also I am not young anymore (46). I am expecting a full recovery but it got me thinking about the stress that paddling can put on the shoulders. Rowing doesn't have the same appeal, but perhaps it is easier on the joints?

(Interestingly the Australian-made Edon TS515 looks like a great way to learn to row, and is about the same size as my 17' ski so I could store it similarly). But hopefully it won't come to that.

I have rowed nearly every day and competed for eight years. I started paddling surfskis about one year ago and competed in three races last Fall.
Both are very low impact sports if you compare them to running but you can still injure yourself pretty severely if you're not careful and if you don't listen to your body. Most are over use injuries.
On the other hand neither of these sports are weight bearing and some impact and weight bearing activities should be part of your routein.
Rowing is generally hard on your lower back and paddling, because it tightens your core, tends to prevent lower back issues. So far, it's working for me.
I've had sore shoulders, injured shoulders, injured biceps tendons, had MRIs of my shoulders and lower back (my lower back MRI looks like a train wreck but I, very luckily, have no symptoms), done rotator cuff exercises, etc., etc.
Right now at age 70 I am pretty much injury free and am planning on 2014 being a good competitive year for me in rowing and in paddling although I am a slow paddler and usually near the back of the pack.
I row 6500m/day and try to paddle about 8000m/day as I have the time to do it since I am retired.
I know I must listen to my body and take days off, or train more carefully, when I feel some stress or strain coming on. If you're going to be very active and train pretty hard no matter your age you're going to have pain and/or soreness in some part of your body a good share of the time. You just have to manage it. The older you get the more injury prone you become. The alternative is to get fat and park yourself on a couch eating Bon Bons.

as 1xsculler points out, I don't think impact is the right measure, training on the river here last year I saw a 4 person rowing skul hit a kayak, and nobody descibed it as low impact also an oar in the back of the head as a rowboat passes too close is moderate impact, it would help if the faced the way they were going. and training in the dark we had a 6 person outrigger (waka) hit and ride over a kayak with no injuries but very loud impact, its a busy we river we have.
back to the initial, rowing is hard on the lower back and does put a lot of load through all your lower limbs
paddling puts a lot of load through your shoulder joints due to the range of movement your arms go though, combined with the movement of water waves etc moving your paddle in diff ways esp if your falling off, and climbing back on
every sport has a risk reward equation for you to make in most cases the risk can be lowered and reward raised by getting some advice/coaching etc specific to you.

I'm 68 and paddle both sea kayaks (Taran 18 and Nordkapp) but spend most of my time paddling my ski these days, averaging 5 days per week. I race regularly (mainly flat and semi open/harbour races) on the Stellar in races of 10 to 20 Km mainly. Initially I had some shoulder problems, but since improving my technique to use my core and properly rotate (ie no "arm paddling") my shoulder issues have gone.

owenw, I applaud you for going those distances at our age. Since, out of personal interest, I tend to focus on the extremely tippy skis I expend too much of my energy keeping my ski up-right to paddle that far. If I can keep having B-days and don't get any bad news from my Dr. I hope I can work up to your distances as all of my home paddling is done on the same conditions you describe.
On Maui the new V10 is perfect for mild ocean stuff and I try to paddle about 8000m every other day. If I ever try bigger ocean conditions there I'll rent a V8.