An i7 will be slightly better, especially the newer generations which have more built-in cache.
My CPUs combined mine me 1 XMR every 160 days according to Whattomine. I am hodling hoping that XMR will moon one day. Besides the rigs are GPU rigs. Main crypto income is from GPUs.
Not free electricity, but all my CPUs are part of GPU mining rigs and the rigs have already paid themselves. So instead of mainly sitting idle, they might as well support the Monero network with some mining :-) . With a combined hashrate of 13-13.5kH/s they make $0.7 - 1.0 a day at current prices.
Also, I like that everybody else is dismissive and negative about cpu mining. Means more profit for me ...