I recently went through the whole computer upgrade thing, and here is my take. Firstly, always get the highest speed CPU you can, no matter what you intend to do with machine. Second of all, in reality, depending on what your mixing style is i.e. a large amount of plugins used versus not many...the most important thing to consider when buying a new PC for creative work is ram, graphics ram and future proofing.
For me, I forgo large bulky desktops in 2010, probably a bit too early in the digital timeline, but now...I would never buy another desktop ever. There is just no point...sorry I know this is a question about i5 v i7...but desktop v laptop is an equally pertinent question.
Since purchasing my new laptop only some 4 weeks ago, with the uber specs of 32 gig ram, 6 gig graphics ram, i7 8gen CPU (4 core), with a USB c 3.1 or the latest thunderbolt for future proofing, and finally internal SSD drive and USB 3 SSD drive for audio and film playback...these are the specs I recommend for anybody looking to get a new P.C for creative work.
For what it is worth, my computer is an ACER Nitro PC, and I use my PC for both audio and film editing. When I use it for audio, I can now use Sonar's 64bitfp mix engine which everyone should use, and I also tend to record and render 64bitfp audio files. That's another discussion ;). Film editing though is where the proof is in the pudding...what would take 4 hours to render on old laptop, which had specs only two years ago that most would have creamed their jocks for, now took 50 mins and less. My film renders are effect heavy and rendered at the highest level, two pass variable bit rate etc. And the best part, my laptop only cost 2500 Oz.
Ben