Again this is a matter of taste in some aspects....as u may find on course is that the R core is more of a new fashioned transformer amazing only ...in my view of course only by the hi packaging capability an so by the hi current capability ...and in conclusion high efficiency per wheight.To that u may add if the ergonomics matter more to you than the sound ,the low profile and as other say good looking...bleah.
For me the king still is the all mighty torroidal...big sometimes pretty hard to manufacture but the results are no to be matched...low noise ...low overall magnetical loss and depending on the power that must be obtained up to 95% efficiency.
Overall transformer design is quite a complicated science ...coz there are many aspects to care about....overall power range and time usage for the stable current flow where thermicity is a real pain in the *** ...type of isolation used.The japs use preventive measures in many of their audio products...exept Accuphase...Nakamiki ..and some other really high end manufacturers. I use almost all the time torroidal based PSU wich i manufacture ....and i can say that the results exceeded my expectations.If interrested in the overall calulus....i may privide u with some interresting facts.
Toroidal transformers have good regulation and low output impedance, but OTOH high interwinding capacitance. High interwinding capacitance means wide bandwidth coupling between the primary and secondary windings. This may not be what you want in an audio power supply, as it implies that all kinds of noise as well as 50/60Hz can pass through the power supply. If you decide to use a toroidal transformer, I would specify an additional electrostatic shield between the primary & secondary windings, as this helps reduce the primary-secondary noise transmission issue somewhat.
EI (frame) core, C-core and R-core transformers usually have split-bobbin primary and secondary windings, which cuts down on interwinding capacitance and makes it considerably more difficult for non-50/60Hz noise components to pass in and out of the power supply. Here you can also specify an additional electrostatic shield between the primary & secondary windings for less noise transmission, but this is not as important as it is when using a toroidal transformer for power supply applications. OTOH, EI, C and R-cores are usually physically larger and heavier for a given VA rating (at a given flux) than a toroidal, and this can have a direct impact on the size and weight of the amplifier chassis.
At the end of the day, it is possible to make a good-performing and good-sounding power supply using toroidal, EI, C-core or R-core transformers. But the cost and implementation issues will differ if you are trying to make any sort of optimized design.