both sound great to me.
With all other things being held equal:
5s + 1515/1Y:
- expected rpm @ 18.5v = 40,700
- battery is cheaper,
- will run slightly warmer (but not hot),
- has slightly less efficiency, and run time. Which you may or may not notice, depending on how critical you are.
6s + 1515/2.5D:
- exp rpm @ 22.2v = 37,740
- more expensive battery (you could always buy another ma 8000mah lipos and wire your existing one in series, so maybe not).
- will run slightly cooler, be more efficient, more run time.
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