I was previously trying to make the decision you are making right now until i found a brewery that blew my socks off. It has some key advantages that neither the BM or the B3 has.
It's fully electric, which I didn't really want at first, but came around. Electric is a lot cheaper per brew and you never have to refill a propane tank. I live in New York and had a hard time imagining wanting to brew in frigid weather (it was 4 degrees last night). I have read a lot here from people who custom fab'd both B3 and BM clones as well as brutus clones and many of them said that if they did it again, they would build an all electric rig. Point is, 'don't fear the low density water heater element'! It doesn't scorch the wort.
Anyhoo, check out the vids. They are of the semi-auto system, but the full auto is just -punch in your brew data, add ingredients, put the sparge arm in place, and come back later for wort chilling.
Key benefits are:
-one thing to sanitize- ball valve after boil kettle
-multiple peristatic pumps- which are like medical grade and he makes them
-so, you don't have to worry about pump priming, etc.. like other pumps
-much less $$ for full automation compared to B3 or BM
-bigger kettles -17.5 gal (not compared to the 20 gal B3, though)
-4 weeks turnaround build time (compared to 12 weeks for B3
-automated sight level gauges
-automated burners/elements- pretty much impossible to have boilovers
-automated timing/brew schedule
-no worries about pilot lights on burners (B3 doesn't even come with pilot lights
-the floating sparge system is pretty cool and prevents foaming in the MLT
-more capable of step infusion mashing if you want
-even a sprayer included to wash things off (powered by the pumps)
-"extremely" low profile design- 56" x 22" x 26"- especially for the volume (sits up on a table)
-more sectioned and easier to break down/move/ship
and for me,
-manufactured n the Hudson Valley an hour away from my house
Only things besides ingredients that this doesn't come with are a water filter and a wort chiller, which are options. Also, the temp logging box and PC software is an add on option, too, but only like $100.
The BM was always a little too pricey for me, so I never seriously considered it. I did consider buying a B3 and automating it at a later date. But for $3700, the semi-auto brewmation is as automated as a B3 or a Sabco and the full auto is even more automated beyond that. I am going to go to his shop and check it out in action before I buy, but I have already decided really.
I'm not affiliated with him, but just though you should check it out. Maybe you'll be as impressed as I am.
Can't imagine either of the others being any more repeatable/consistent/reliable that this one and this one maybe even more so than them.
Also, nothing against a Sabco, but they sure have gotten a whole lot of mileage/free press out of breweries like Dog Fish Head using them for pilots. B3's are similarly utilized as pilots by equally illustrious breweries, so don't let that influence your decision too much towards the BM. It's just well played marketing.