SRAM Red is lightweight and saves some money, but it has two big problems: an uncomfortable lump in exactly the wrong place on the brake hoods, and its sloppy unidirectional shifting along with the “lose a gear” behavior when one is in the last rear cog. No WAY would I buy or use SRAM Red— it’s irritating as hell.
SRAM brakes also disappointed me with an annoying vibration (brake pad glazing involved, but DuraAce does not have a problem with that). I have never experienced vibration problems with DuraAce (same wheels and pads), even with glazed pads.
Regular (cabled) Shimano DuraAce has a big problem with my winter gloves: I found I had to attempt a shift 3 or 4 times; the levers are too small, making it a fumbling affair to shift. An ergonomically challenged design for winter use.
With Shimano DuraAce DI2, I get perfect and FAST shifts every time (!!!), and I can shift with ease even wearing my winter gloves, even with a glove liner and wool outer glove (thick).
Note that Shimano has a less expensive Shimano Ultegra DI2, but I hear that the shifts are not as fast. Still, I bet it’s darn good, so if the budget is tight, Ultegra DI2 might be your answer.