I agree with Geoff completely. But, everything being equal, I think your best bet is keeping the OS that is on the new computer as Geoff pointed out and just reinstall your programs. Much cleaner that way.
If your old computer were actually "fairly new," then you wouldn't need a new computer. But, it is, as you say, old enough to be replaced and, hence, it is probably time for a reformat anyway. Unless I am missing something.
