Splintertalk has the look and feel of steemit so it is more familiar to use. The forum is slow (loading and responsiveness) so it is not really user friendly to me. And it does not show up nicely on mobile, which I am using most of the time to browse contents on the blockchain.

Contents on the forum has more permanency therefore I will usually post questions on the forum as it stays visible for a longer period and hopefully gets responses from readers. Splintertalk, like steemit, contents usually just last for one day before it gets bump down to the black hole.

So I think both serve different purpose.