the 2 may actually be the same technique. The difference is, that Tobi definitely has had much longer training with his, and certainly seems able to use his for longer periods of time. Kakashi is still learning his, and can only use it a little bit before he's exhausted from it's use.
Or they could be completely different.
Tobi's is without a doubt an alternate dimension.
Yet Kakashi's could just be a pin point of intense gravity. which would explain why it doesn't suck things in whole, but rather breaks them apart.
of course I'm not sure how that theory holds up if you account for the pain attack on konoha, where he used it to avoid the nail sent at him from yahiko's pain.
They certainly LOOK similar when being used.
It would almost seem to lend some creedance to the theory of Obito having an older brother who escaped the uchiha massacre. except that I'm pretty sure he'd be blind from MS by now if that were true.