This is a hard question to answer, I like both of these machines! I think the Ultra 60 would have the edge when it comes to performance. The Ultra 60 was a high quality machine and it has very good reliability. I've seen many Telco's running switch managment applications on Ultra 60s because the operations staff could depend on the machine to give excellent availability. Getting a bit old now though. The Blade 100 is a nice little machine, it can support fairly large generic disks and the IDE controller will allow you to use a broader range of " off the shelf " removable media eg. CD-RW or DVD-RW, but the slow disks and CPUs small external cache may inhibit performance of some applications.
Advantage of Ultra 60 is that is actually a server mobo in workstation case, there is corresponding server model, I think fire 220 or something like that which uses same mobo, memory... If you will use it primarly as a high traffic server it may be good idea to go for Ultra 60 becuase you can have multiple CPUs and there is SCSI bus, but it is an old machine with old design which also makes a lot of noise.
If you want to save money and get decent box get Blade 100, 500 Mhz and easily you can overclock it to 550MHz (10% CPU speed increase for free!!!) Close pins 1-2 on JP3 on mobo (by default they are all open on 500Mhz models) and you are in business. If you are lucky you maybe able to get to 600Mhz, but I am not sure how stable it would be.
I just got one blade 100 from ebay for $100 and it came also with SUN PCI IIpro (celeron 733Mhz and 128MB of RAM) so I'll install linux there. Can't beat the price, 2 computers in one for $100 and 10% free CPU speed increase on top of that. Thank you SUN!!!