Dreamcast's graphics had more video memory, better Anti-Aliasing and other PC-like features that offered it an edge on certain games, especially exclusives, but that was as far as it went.
PS2 was able to produce more things on screen i.e. polygons, effects etc. And seeing as it was out longer and had better game development support, you had great titles like Shadow of Colossus or FFXII showing what it was capable of.
One big example, is to compare the DC and PS2 ports of Quake III or Unreal Tournament. DC versions "looked better" but lacked the bigger maps due to memory limitations, whilst PS2 versions didn't look as good but had all the bigger maps and same standard content as the PC versions.
So yeah, they both had their advantages and disadvantages. Seeing as the Dreamcast was out first, obviously it will have some better looking games. Shenmue being an example of something like 7 years development beforehand and was originally a Saturn title. But once the Dreamcast stopped production in 2001, from there on, the PS2 had some major gems of its own as it carried on, especially near the end of its average lifetime i.e. 2005 onwards.