The 7% MP saved on average for every cast is more than the amount you get back from autorefresh in the same time period given that you're spending enough MP though. Conserve MP tends to be more useful for endgame stuff that might require you to cast big spells more often.
If you don't want to rest, like you're PLing or something and just need a constant steady stream from 2 tics of refresh, /SMN is way better though.
It's been a while since I've done hnm, but if you're spending 300mp/minute on average you guys aren't gonna last that fight lol.
Really the best sj is probably rdm. 15% fast cast/7%recast is nothing to scoff at, and phalanx can be equivalent up to -20% dt; two things that can actually prevent death.