Why not both? That's what I'm doing.

Tbh, its depends on what you want. In terms of contents, NA will get the contents sooner or later, except the QoL ones which are much earlier. I'll list the pros and cons below for your reference so that you can decide for yourself.

JP
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Pros:
-The earliest version to get any update
-More servants
-Very generous SQ giveout

Cons:
-Unable to understand the story if do not understand Japanese
-Probably reaching endgame in 2 years or so. (Last Lostbelt or something)
-----------
NA
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Pros:
-English Language
-QoL updates arrived faster than the corresponding schedule of JP

Cons:
-Some features in JP not available yet in NA.
-Content lagged by 2 years compared to JP

but im using ipon