Here's the thing: the $100 promo is quite good if you've got even three lines, because of the extra stuff Sprint adds on top for new customers: 2GB of extra data per line, and no access fees until 2016...and I'm sure they'll have some new promo to take advantage of at that point. By comparison, T-Mobile's offer drops to 4GB around the same time frame, though I think TMo will actually let you use their new plan as an existing customer.
Now, if you're talking about existing customers, Sprint loses out: the access alone are $100 for four lines. If you're comparing to VZW and AT&T, you get double the data at $160 for four lines, but for those same four lines there's no price point between 4GB and 20GB the way the access fee is structured. Then again, VZW and AT&T pull the same crap at the lower tiers, except worse (you have to get 2GB of data on VZW to actually come out cheaper than 10GB).
Really though, Sprint is pretending that it isn't competing with T-Mobile. Again. Yeah, have fun with that.