Jasmine was always a challenge, but I would generally have one of my pokemon know dig and over-train it so it could take out that steelix. the magnemites and their thunderbolt was harder for me, because a 'mites special is so high. for me, whitney is the toughest, generally because you don't have a great type advantage that early, HOWEVER, you could trade for that machop in the mart and train it excessively.
in conclusion, that miltank is a pest. whitney was harder.
if only you had a magmar with dynamic punch and fire punch... lol you'd take them both out no prob. too bad dynamic punch isn't available that early. i thought chuck was tougher than both of them though. that dynamic punch hits more than it misses with him, for some reason. and my pokemon ALWAYS hits himself in confusion.
with the right team, neither gym leader is tough. none of them are, actually. they are just annoying! rollout, milk drink, steel tail, its all annoying, but it can be handled somewhat easily.
just my two cents.