AFAIK, there are no AM2 2.7GHz Semperons, they top out at 2.3GHz with the Sparta core.
There is a 2.7GHz AM3 Semperon but I doubt that is what you are looking at.
Probably you are mixing up the model number (2800+, 3200+, 3500+, ect.) with the clock speed.
Regardless, I would defiantly recommend the AM2 system.
Socket 478 is 100% dead and finding RAM and AGP GPUs for it will be a major (and probably expensive) PITA.
The AM2 system, on the other hand, could be quite easily upgraded with a more current PCIe GPU, more DDR2 RAM or a new CPU (make sure to check the motherboards support list).
As with the above, if you are paying for either of these systems think long and hard before buying.
Both are seriously outdated and are worth next to nothing..