Purely from a hardware stand point of view:
htc hero uses an old processor Qualcomm® MSM7200A™, 528 MHz where as n900 uses OMAP3430 which is currently powered at 600mhz and can be geared upto 1ghz.

Also, the TV-Out feature.

All-in-all, apart from the form factor (this is a personal choice), N900 is great and undoubtedly superior than the hero!