So, everything I’m about to say is based on my experience, so your mileage may vary, but I think (hope!) it’s pretty reasonable:
Though it’s possible to go pretty durned fast on 100mm cranks at 1:1, I am pretty sure the general consensus here (and my opinion, too) is that the 150mm cranks at the 1.5 ratio are hands down going to be faster in just about every situation, assuming that you can get used to them. (And not to worry, if you have them, you most definitely will get used to them!)
Of course it’s the same “total gear ratio” or whatever term was used long ago to describe the mechanical (dis)advantage your foot has over the ground, but the 150mm cranks at 1.54 gear ratio are going to use a whole lot more of your leg’s range of motion with a whole lot less mad-spinning cadence. (There’s a reason road bike cranks usually range between 170 and 175mm!) If your experience will be at all like mine was, your pedal stroke will be smoother, your force on the pedal will be more even, and your legs will generally love you on the 150 cranks compared to spinning the 100s.
In some cases, on really steep grades (whose steepnesses depend on rider comfort I suppose), the 100mm cranks in 1:1 can be faster than the 150mm at 1.5:1 simply because keeping balance is easier when torquing the pedals hard up a hill if you’re not having to use the uni frame and your body as a torque arm for the geared-up hub (having to pull the seat back between your legs when pushing the pedal down!). It can be easier to mash 100mm cranks up a hill than to mash 150mm cranks while geared up, just because of the way balancing works in high gear. But if a hill ever, ever, ever gets steep enough for this to be the case, you long-ago would’ve shifted the 150mm geared setup back down to 1:1, and begun to use the 150mm cranks at 1:1 in “ungeared mode,” which has a huge advantage over climbing such steep grades with the 100mm cranks alone. I think that’s why John mentioned that the 100 cranks would be awesome if it’s “flat enough:” because you’re basically locked in “high gear,” with “high gear” meaning teensy crank arms.
So, basically, yeah, the geared 150mm setup is the way to go if you want to get around as fast as you can, but it’s really expensive.
I’ve seen some people do things on ungeared 36ers, too, that are just nuts to me: for example, in July’s races here in Northern California, Scott and Martin finished only a couple of minutes behind Corbin on his geared 36. So ungeared 36ers can seriously haul with the right riders.
I would say that if you’re on a budget, the 100/102 cranks like John Foss is recommending are a no-brainer, as the extra MPH wouldn’t be worth the extra $1500 or whatever a hub costs these days (over a conventional hub).
If your budget can include a hub, then I soundly believe you won’t regret getting one. Ride it and you’ll be hooked! It’s seriously like having two 36ers in one, with one of them being so fast that no other unicycle (except mayyybe a geared 29) can touch it!