Oh, definitely rounds. A new set of nickel rounds (I use D'Addarios XLs on both my slappers) has no equal for slap. Stainless CAN work, but you tend to get a very bright sound that, coupled with the zing and pow you get out of thumbstyle, can be grating. I've also found all-stainless sets to be very scooped-sounding; lotts highs, good lows, no mids. Even though some slapmasters scoop their EQ, you need SOME mids to define the sound. Don't believe me, try slapping with the tone control off; it's just a bunch of mud. Add treble to that and you have unpleasant mud.
Now, that unpleasantness changes after the bass has had its strings on for about a year. Good all-stainless strings last a VERY long time before dying completely, and when they dull slightly you get a great groovy sound that slaps far better.