I would stick with the 1/2" boring head if it suits your needs so far as capacity (diameter) is concerned, bear in mind that a head with larger holes will likely be physically larger than a 1/2"head, diminishing the height available under the head/boring bar down to the table. The only reasons that I would go for the 3/4" head would be that I wanted to use 3/4" diameter tooling and/or I needed the larger diameter boring capacity that the larger head would likely have.
I doubt that you would find much difference regarding the shank issue with a 1/2" sized boring head, perhaps, yes for the 3/4", it may make a difference in ridgidity (lesser) going with a straight shank if you were doing boring with heavier cuts and large diameters and tool overhangs. With the straight shank on the smaller head, tool change would be more convenient due to not having to pull that long R8 shank out of the spindle. The mill that I had at my shop before retirement had a R8 spindle (a Induma 1-S. a Bridgeport type machine, but heavier) and it was a pain when changing tools, frequently having to lower the table to accomplish the task; in my retirement shop I have a Fray vertical mill from the 1940s and a Brown & Sharpe universal with vertical attachment, both have 30 taper spindles; much more convenient to change tools!
I have many (about 7) boring heads that fit the mills, up to 3/4" hole size, and find that I mostly use the 3/8 and 1/2" heads.