A few weeks ago on eBay, 2 Guru Islands went for almost $100 each. The price is back down to around $50-60, but Summer Islands seem to always sell for less than $40. There are some BIN prices on eBay for over 50, but they don't really sell. I got 2-3 Summer Islands a few months ago for around $35, so they aren't too hard to get.
In my experience, Guru Islands are just as rare as Summer Islands. There are 11 Summer Islands on eBay BIN right now, but only 1 Guru Island for auction. There is less demand for Summer Island- so while it may technically be a bit rarer print run wise, there certainly is a decent amount of supply out there. Most of them stay unsold, while Guru Islands go pretty fast.
I think at worst they are fairly equal, but I do prefer Guru for the art and black border.
Guru!