It depends, in some cases tile flipping is more useful. It depends on where you save more space.
The major drawback on the SMS is that you can not really design tiles in such a way to easily swap colours, and as such saving space on the rom. On the NES you can do that, because you got 4 bg tiles. From what I understand, if you made the same block 4 times, using different palettes. That would still take up space in one of the tables (nametable or something, Kasumi would know), but only 4 tiles in the tiles (assuming a 16x16 pixel block here, because of the tile colour attribute size)
If you wanted to make the same thing on the SMS you can have the colours to do that for sure, but it would be 4 times as many tiles, so quite wasteful in that sense.
For straight pixelling the SMS is probably easier, once you get a good palette from the 64 possible colours worked out. Though I have to say I personally find the NES a lot more interesting to work with. But then again I really like minmaxing palettes etc (which is really crucial on 16 bit systems like the SNES, MD, etc.)