Although I didn't do a runtime test between the M3 & the M3 Pro, from trying out both I found the M3 Pro to have better battery life in low power mode than the M3 in "regular" mode. It also doesn't lag or stutter like my M1 Pro on low power mode.
M3 has a slightly smaller battery. However, the M3 has insane battery life, too, so whether it's worth $200 or so to run the M3 Pro in low power mode (if you don't need the power), is really up to you. Personally, I wouldn't necessarily "pay up" for the Pro chip just to run it in lower power mode all the time. But if you think you might need that power sometimes, it could be worth it.