I'd rather share data through my iPhone. On my plan, that's a savings of $10/mo x 24 months = $240 + $129 cheaper for the iPad, which you could spend on more storage or go towards a new ipad.
Technically, you're not locked into a contract with the iPad so the $240 for 2 years is not mandatory.
If you do decide to get the iPad on a 2-year contract, last I checked, Verizon and AT&T both give a $100 discount on the device so that's really just $240 + $129 - $100 = $269 or an additional $11.21/mo over a 2-year period. Cutting down on Starbucks once a week would get me that savings. Of course, whether it's worth it or not all depends on your usage. I use cellular data on the iPad quite heavily (and even use it as a hotspot) so to me, it's well worth the convenience of having built-in cellular on the iPad.