while i have not had the oppurtunity to test the evo i would say it is more likely to be louder as it is media centric with the kickstand so it probably has decent volume but this is just speculation.
I thought the EVO was loud and good quality compared to my Tilt and Nexus one. However the sound quality on the Nexus S is superb. However I no longer have the EVO to compare so I can't say for sure which is louder. I can say that I never used the Nexus S with the volume more than 60 or 70%. It's way louder than Nexus one.
Samsung always makes the best speakers, only problem is that they are muffled whenever you lay your fingers on top of the speaker grill. It seems to be aa problem with most devices.