I will add my 2 cents for what its worth. I replaced my indoor Benjamin wood oil combo boiler with a CB 6048, located outdoors.
The Benjamin had 40 gallons capacity, the CB 6048 has 390 gallons. These are both relatively inefficient boilers and not gassers.
The Benjamin, with small volume of hot water could never keep up with the heat load of our 4000 sq ft home and shop....the cooler return water just could not get hot enough, fast enough to keep the temps in my radiant floors warm enough to satisfy the load. We had to heat by zone with timers to even keep comfortable.
The CB however, with its large capacity, is able to provide enough hot water, 180 degrees, that more than meets our loads...and all our DHW, (Benjamin never did DHW).
In a traditional OWB, water volume means a lot....as there is a temp diverting valve that will start to operate at 160 degrees, and basically shuts any flow into your heating system at 150 degrees, and sends it directly back to the boiler....this is done to prevent shock to boiler...without sufficient capacity, and a high load, your return temps would drop so low that the thermostatic diverter valve will kick in and you will start to diminsh hot water entering the system.
Gassers run on much hotter fire temps and can heat much more consistently. You run short hot fires and utilize all the efficiency thru storage of some means...or you need to run a few short hot fires a day to keep up....
So, gassers with storage or OWB with higher water volume capacity perform to the best of their respective efficiencies. (does depend somewhat on your loads and climate....I am in Northeastern Ontario...and we have long cold winters.
OWB are very common in the rural area I live in....we looked at gassers, but decided on the CB 6048. I know a lot of members here are not big fans of OWB, (smoke dragons), but for us...it is the best solution by far. We have unlimited wood on our property, we get great long burn times, coldest winter days at -30 degrees we stock twice a day, (and we dont fill it up either), at this time of year, we get 24 to 36 hours of burn time....
Our wood usage is not horrible either, maybe 1 cord more than what we used with the Benjamin...so perhaps 8 to 10 cord for a full heating season from September thru to end of May...yup...long heating season eh....
Not only is the wood consumption reasonable, maintenance is low on the CB,, (clean it once a month isntead of every 2 days on the Benjamin), wood handling has been reduced drastically, no more splitting, piling, and hauling and stacking in the shop. Now we just skid our logs right to the boiler location, buck em up, stack en and ready to go....My family and my labor time to process wood for the CB has taken weeks off our wood prep time....weeks
Good luck with your decision...
Mike