I think there are generally 2 schools of thought on this matter, and most of the views that follow this post will generally fall into 1 of these 2 categories:
1) "Get the 1.42 because you can't upgrade the processor later."
2) "The difference in speed is not perceptible, get the 1.25 and spring for 1GB of RAM."
My personal opinion is that I would prefer to get the 1.42 with 1GB RAM.
I think that with 1GB of RAM, your Mini will purr along nicely irrespective of the processor speed.
Good luck with your purchase!