The i7 cpu will show an improvement in performance over the i5 cpu not only due to the increased speed but also the improved technology of the cpu. In addition to the design of the chip and the speed, the i7 has 4 MB of onboard cache vs 3 MB for the i5. Turbo boost, overclocking, is significantly greater on the i7.
If the i7 runs hotter than the i5 is not important as the cooling system, the fan, has been designed to keep the chip in a safe operating temperature range.
Resale value of Macs depends a lot on the condition of the Mac...keep it pristine and working well and the value will stay high...a quick look at sources like eBay will show new vs used prices and what one can expect.