Hiveworks and Tapastic are pretty different services.
You get an account on Tapastic which lets you publish your comic. You don't really have your own website, you have space on the Tapastic website. Tapastic doesn't promote all its creators -- there are ways for creators to be spotlighted, but they might literally might never happen to you. And Tapastic markets primarily to its audience -- you are unlikely to run into Tapastic outside of the Tapastic website.
Hiveworks does offer hosting, but in a completely different way -- every webcomic has its own website, and those websites have a series of ads on them that direct people to other Hiveworks comics. In the same way, other websites' Hiveworks ads will now sometimes direct them to your comic. (And in that thread Kaykedrawsthings linked, someone mentions that Hiveworks spent a pretty significant amount of money marketing their comic in its early stages, and talks about how their traffic was affected by a switch from Tapastic to HW)
I think it'd be hard to compare in terms of which one's more viable -- it's also a matter of which approach you prefer. Tapastic is a closed audience and it's notoriously difficult to coax that audience to other sites, but it makes no demands of its creators in terms of quality or schedule or fully planned story -- you can do what you want with your space. Hiveworks is not a closed audience and actively promotes its members -- but it wants you to promise a regular schedule of updates and sign a contract.
But there's no harm in applying. You can always apply, and if you're accepted, discuss all this stuff in the Skype interview where you hammer out your contract, and change your mind if it doesn't sound right for you!
