MCU? Mjolnir more so focuses his powers, yet Ragnarok proved hes just as powerful with Mjolnir. Strength wise we saw him battle Hulk evenly without Mjolnir or Stormbreaker, and even take the full force of a dying star without either weapons. Yet both weapons gives him flight.
Comics? Thor doesnt have Stormbreaker, it belongs to Beta Ray, and Mjolnir, I believe is the slightly superior weapon. Anyways, with Mjolnir Thor is still just as strong and powerful..hes just absent flight, and greater control over lightning (which is stupid).
Thor has shown self-propelled flight without Mjolnir too, but it's very inconsistent, sometimes he can, sometimes he can't, more often it's simply ignored.
Mjolnir is also somewhat sentient, unlike Jarnbjorn, and grants Thor protection from curses, mind control and most common spells, so Thor is much more vulnerable to non-physical attacks without his hammer.