Pretty sure the R is also 24mm in the front. FWD cars might be different.
If you believe this random aftermarket vendor, front sway bars are "street car shit" and rear-only is race car
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Most people who've bothered to deal with the front have gone to LCAs with stiffer bushings, so it's hard to get a controlled comparison on NVH.
The main reason to do both sway bars is to gain adjustment, it's like running coilovers you can only adjust front or rear. If the car is understeering after adding a front sway you just need to re-tune your setup (endlink position, dampers, and tire pressure balance).