Allow me to opine about both, as I currently have both.
Actually I have BAT monoblock 150SE's (in an all BAT system) and Mac monoblock 275 IV's (in a second system). When they were the best BAT tube amps, I had monoblock 60's.
I have not had any problems at all with any of my BAT equipment. I don't think I am just lucky. BAT equipment is well designed and well made, IMHO. No manufactured equipment is perfect and there will always be some individual lemons.
Yes, the BAT amps, using the 6C33C output tubes, generate more heat than the Mac's and, as suggested, consideration should be given if used in a small room and/or in a warmer climate.
Unless you get the newest 275 V amps, the older versions have the archaic speaker terminals Mac has used in the past, making the use of spades and bananas problematic.
Annoying but not insurmountable.
That said, now we have a level playing field - both amps having the same power output.
I listen to both but prefer the BAT's. I must add that my 150SE's have been modified with V-CAP teflon capacitors, a very simple procedure to do, thanks to the help of Dgarretson. That has made a huge difference in sound - much more low level detail, improved dynamics, and a deeper, wider sound stage. Others have previously commented on the benefits of V-CAP teflon coupling caps in various units. Well worth the cost. But, even before that mod, the BAT equipment drew me more into the music, which is what this is all about, right?
I emphasize that you must hear both to make a better decision. Both are very popular having achieved widespread acclaim for their sound.
Good luck.