I vote for,
18650 cells and a crap load of spot welding for several reasons.
1) Higher power density
2) Lighter weight
3) Easier to service (replace cells) yourself if built right
4) Easily customisable shape
5) I plan on building my next E-Bike packs from 18650's, so would benefit greatly if you did also.
It might seem/sound daunting building a 30KWh pack from 18650's, but with the right equipment, once your in the groove of building it, I think it would just be a matter of slugging it out every night for a few weeks to get it done, provided a lot of planning went into it first.
You could use some of the new LG or Samsung 3500Mah cells and get away with using far fewer cells than otherwise. These cells can manage 10A each also, so have more than enough potential for high amps draw. Right now I would recommend the LG INR18650 MJ1 3500mAh from Nkon as you can get them for €3.75 if you buy more than 300. You might be able to get them even cheaper with the quantity you would be after...and I will be after some also soon (about 280) for my bikes also.
http://ru.nkon.nl/rechargeable/18650-size/lg-inr18650-mj1.html
To get 30KWh at 120 volts nominal you would need about 32S and to make that 30KWh you would need about 72 paralleled so 2304 cells total....
2304 cells total 
Yeah, that's a crap load of spot welding...but, consider this...
That pack would only weigh in at 107.5Kg
Would be able to deliver 720amps without an issue...probably being ok up to 1000A for short bursts
You could add another 28 cells in parallel for 100P guaranteeing 1000A continuous, and giving you 41.4KWh.
The only catch I see is it would cost about $13000 AUD...before shipping for the 30KWh and $18k for 41KWh...ouch! Although I have no idea what your budget is for the battery, and how this compares to the alternatives so could still be ok I guess.
Hope that helps.
Cheers