Generally, if you are intending to keep the machine for say 2 years, and then resell it, the higher specs will make your resale value higher, by (based on my local experience) 50-100 /$.
If this is your plan, you only need to justify a difference of 200-300 $
Otherwise, it's really a question of how bad your nerves are/how impatient you are. The i5 will run anything the i7 runs, just slightly slower (and in anything except extreme cases you won't note the difference). Usually the human is the bottleneck.
RGDS,