I need to make a dishwasher power cord.

New DW specs:
Min 14g 167f

Current house wiring:
dedicated 20a circuit [12g]
15a split receptacle [w/disposal]

Q: Can I use 6' of SJOOW [300v] & add a 15a plug? [the other end = pigtail to DW]

Q: 14/3 [18a] or 12/3 [25a] Is there any reason not to use 12/3 over 14/3? [most DW cords I see are 16/3]