Trident and SiS have one advantage: As their products usually do not perform as fast as NVIDIA or ATI chips, hardly anybody would have high expectations.
Currently, SiS is releasing new drivers every few days, thus the performance of 315-based graphics cards could increase before the first cards hit the shelves. From the software point of view, SiS provides a timely package including some options to influence performance and image quality.
A problem that both chips have to fight with is the lack of OpenGL support. Of course hardly anybody is going to purchase a graphics card based on one of these chipsets to run professional OpenGL applications. Yet there might be some people who would like to play Quake III or run other software based on OpenGL.
Though both chips are not completely finished, it's already quite clear that the SiS chipset should be the better choice, even if it is more expensive. The flexible memory interface leaves you the option to chose either SDRAM or DDR memory in several configurations.