She has an extra 1.25 second stun (well, if you count her E as doing the same thing as Amumu's Q... even though his is a stun and hers a snare, it's basically the same for ganking.) Her gap closer/snare goes through minions, but has shorter range (just eyeballing on the range issue). Not sure who brings more damage, since her passive adds quite a bit. Leona's ult is ranged while Amumu's is centered around himself, but Leona's ult is only an 80% slow instead of a stun if you don't hit directly with it, but it has a much shorter cooldown.
So she's really a better ganker, but her clears are just abysmal. --- "That is why war is so tragic. To win means to make victims of your opponents and give birth to hatred." - Kratos Aurion, Tales of Symphonia
11/19 masteries. Taking attack speed, minion/monstore damage bonus and magic penetration masteries in offense and just general jungle-tank masteries in defense. 10% attack speed through runes, 7 AD, flat armor seals and magic resist/lvl glyphs. Also tried a variation that gave me 25% attack speed total for faster clears.
Trust me when i say - you don't want to have a Leona in your team as a jungler.
The issue with Season 3 jungle is - if you've got a slow clear time, you'll never, ever get good ganks off. This is unfortunately very true for Leona. Her clear time is atrocious and no matter what skill you take first, you'll still be taking too much damage to even get red after you do your blue-side jungle clear. You can forget any and all early ganks with her. Another issue is that her skills don't do much damage, but you still need to use them whenever possible to help out her clear time. This creates an interesting situation where you're pretty much forced to take 9 in Utility to have mana to spam those skills(otherwise, you'll run out of mana really quickly), but if you do that you'll take even more damage(or clear slower) and will have to back sooner.