Without specific knowledge of what your goals are for the system, ie. machine learning computation, website hosting, video games etc. All of the (3) I mentioned have to have a specific aspect of your machine optimized!
1. Machine Learning Computation: generally you will want a faster CPU and a better video card
2. website hosting: you would want a ton of RAM
3. video games: a great card will really help.
To answer your question: On a budget I would go a better/faster CPU
why? #1 Mac's use a variant of unix at it's core. Hence it is quite stable. #2 modern Macs use ssd(s) hence read/write speeds are quite high. EPIC! #3 on chip video processors ie intel 640, 650, 655 are bad at all.
SO ON A BUDGET GET THE BETTER CPU!
p.s. more more is better than less cores/ faster cpu is better than slower cpu/ better family ie i7 is better than i5 - i7 has capabilities
that the i5 doesn't
I hope this helps. jana