Cheap attack speed+movespeed. You're now at ~510 m/s with the active, and as soon as you get it you're able to fit auto attacks in between omnislash attacks. With the recent crit% buff, you also crit more often in between which deals stupid amounts of dps, plus it allows you to farm very, very fast through the jungle, and hit ancient stacks too. I don't just think of team fight or pick off utility, going SnY into MoM allows you to farm at a VERY quick rate, cheaper and more effectively than anything else I've seen. This lets you hit closer to 500 gpm even without kills, and gets you your next few big items like skadi/basher/bkb even faster. I shamelessly stole this build from a 5.7k jugg player I matched with for this reason.
Don't bother with midas unless you can get it at like, 4 minutes with zero harass in lane. Even then, basi + treads is often better I've found. It doesn't let you farm as quickly as a ring of basi + treads. You can't clear jungle stacks + lane creeps with a midas.
I was going to half ass this, but here's an in depth analysis and breakdown of why SnY + MoM is better than mjolnir.
Mjolnir costs 5700 gold
SnY + MoM costs 1800 + 4100 gold (5900)
Mj is cheaper by 200 gold
Mjolnir has a 2,000 and 1600 gold buildup items, SnY + MoM has a 1000 gold buildup item at most.
Mj is harder to build up, and you do not get the benefit sooner of having cost effective buildup items. This means that you WILL farm slower.
Mjolnir gives +24 damage, +80 attack speed, and a lightning proc which is 150 damage at 20% chance, or an average of +30 damage each auto attack per target in an area up to 8 targets. It's also a unique attack modifier, so if you want to build something like skadi, deso, or lifesteal, you have to worry about it proccing and overriding your other UAM's.
Sange and Yasha give
for +32 damage, +32 attack speed, +16% move speed, plus +16 str/agility (armor + tankiness) PLUS it's maim passive, which requires no mana and is a pseudo disable slowing attack speed + move speed unlike MJ active.
Mask of madness gives +100 attack speed, and +30% move speed on a 50% uptime.
Total for uptime is 32 damage, 132 attack speed, +46% move speed (not stacking), str/agi stats, 20% life steal which you CAN use to heal up in omnislash, and psuedo disable.
mjolnir will, in total, give more damage based on it's passive procs at the cost of 52 attack speed, a massive amount of extra move speed, (355 with treads to ~510) and a much harder buildup. If you die while obtaining that 5700 chances are you'll be set back a whole lot, and wont have the option to buy cheap parts before death.
The negative side of MoM is negligable, you'll be invulnerable for most of it if you position your ult right, which you better do with that near max move speed, then immediately spin out afterwards to avoid magic damage, get back behind your team, and reposition to start right clicking/putting up your healing ward.
With the SnY + MoM build you'll be able to run all over the jungle, have sustain to never run back and forth from the fountain, and farm ancients, and take roshan. You can not do that with a mjolnir, you are way too slow, rely on your healing ward, and have no tankiness to speak of.