I would definitely go for 30K for a number of reasons:

1. 30K gives you plenty additional legroom as it is at the overwing emergency exit and you can get out of your seat without having to climb over the passenger in 30J.
2. 23A is a bulkhead, but has the wall to C-class immediately in front. If you are tall, you even have less space for your feet than on any other normal seat. You can also not get out easily without bothering the passenger in 23B.
3. On 23A, it is not unlikely that a mother/father with an infant will be sitting next to you. In addition to the mother/father you will have an, albeit small, but potentially noisy, additional person next to you and after takeoff a bassinet that will be attached on the wall in front of you. This will restrain the small space that you have even more.
4. On 30J/K (or 30A/B), there will be no infant and bassinet allowed. 30D/E/F/G are bassinet positions.