@Joe Legan
I have played Donnybrook and bought the At Sea expansion.
You are quite correct in saying that Donnybrook doesn't have the online community or range of models/materials to support it, so one can only really compare the rules – which is always going to be subjective
Whilst Donnybrook is a fun one off game, and fairly traditional in its approach to the core mechanics, I find B&P offers far more depth of gameplay and long term interest.
The Donnybrook activation system is rather bland (each unit has a card, and activates when the card is drawn – there is no tactics or decision making involved). B&P has you play a card from your hand, the higher the card the fewer actions you get, so you effectively trade initiative for actions – and then decide which unit to activate.
Donnybrook has a very familiar (traditional) turn sequence once a unit activates; move, shoot, charge/fight, rally. In B&P you choose your actions up to the limit on the card you played (1-3). You are therefore again having to make tactical decisions about what the units will do…move, shoot, reload, rally, perform seamanship actions or repair damage. I like game mechanics that force me to make choices, and I find B&P far more innovative in this regard.
Shooting in Donnybrook is a test on a die which depends on the quality of the unit (d6, d8, d10). I quite like this – it comes straight from Beneath the Lily Banners which is a rule set I enjoy. What I do not like in Donnybrook is the way reloads are handled. When the reload card is drawn, everyone on the table on both sides magically reloads their weapons. B&Ps shooting is a bit more sophisticated, and you have to take actions to reload your weapons. B&P also allows defensive fire to a loaded unit when it is charged, and so managing reloads is a tense part of the gameplay. B&P also allows units to half shoot, meaning you can save some of your firepower in case you are charged – which I really like.
Close Combat in both systems is similar (except for the defensive fire mentioned above).
Morale is one of the biggest differences. Donnybrook is traditional and familiar; lose half your unit or lose in cc and you roll a morale test on a table. B&P tracks morale through "Fatigue" – whenever a unit is hit, takes casualties, pushes itself for more actions or makes defensive shots, the unit tests to see if it gains fatigue points. The more fatigue, you start to lose actions and eventually fall back or run. Managing fatigue is a key part of the game in B&P – but in Donnybrook morale is really just die rolling.
To be honest, I was massively disappointed in the Donnybrook at Sea rules for ships. Ships are essentially just another unit that has its own shooting, damage track etc, and they activate once per turn like any other unit (the units on board don't really crew the ship). The sailing rules themselves are basic. B&P handles things differently – the ships are effectively mobile structures – and move in parallel to the normal turn sequence. Units on board have to actively manage the ship however – manning the guns, changing sail settings, taking seamanship actions to perform advanced manouvers etc.
Both games have a lot of historical background and lists of factions. Donnybrook has more geographic breadth (covering everything from Scots Covenanters to "Natives"). B&P covers only the Americas currently, but has a huge amount more depth, with far more differentiated unit types and more detailed force lists within each nationality. I found Donnybrook forces played very similarly – the biggest distinction being the quality of the units fielded. B&P factions and units have a lot more special rules, and different factions/nationality have very different play styles and tactics.
In summary (and in my own personal opinion with which I am sure others will disagree) I wouldn't call Donnybrook innovative or detailed. Fun, but basic and quite traditional in approach. Blood and Plunder is a far more sophisticated rule set, with a lot more depth to the sailing rules, factions and gameplay.