I bought a half decent unit that holds about a quart of fluid that I use filling and draining things like the transmission.
For gear oil I find a hand pump way too messy (and you'll never use the pump again for anything else). Instead - I put a piece of nylon tube on the end of the bottle (which usually comes with a pointy end that accepts a small tube nicely). Then, insert tube in diff and then lift the bottle up above the level of the diff until full. You can do this from above to avoid getting drenched in stinky gear oil.