Hard to say based on JUST those specs.
>DUAL CORE3.4GHZ or p4 2.8ghz
This is the CPU and the first is better
> 2GB or 2gb
This is the memory (or RAM) so both are the same
>80GB or 120GB
This is the hard disk size so the second is better.
A lot depends on all sorts of other factors like the age of the computer, what case it is in, how old it all is, if it all works, what software is on etc.
However based on the basic facts given the first has a better CPU so that is better.
The first has a smaller hard disk but it is easy to fit a new large hard disk, not so easy to fit a new CPU.
Go for the first one.