I'm a big fan of the Tamron 28-75 2.8. I bought a well used copy for $260 about two years ago and it is still going strong.
It's $500 new, so you can have a good lens now and save up for the later purchase of a trophy lens that will make you feel good. http://www.amazon.com/Tamron-28-75mm-Aspherical-Canon-Digital/dp/B0000A1G05
For landscape work you will presumably be using a tripod so you won't need stabilization and you will be stopped down to F8 or so. You don't need the latest and greatest lens. Why spend a lot of money for capabilities you won't use?
The really great thing about this Tamron 28-75 is the small size and light weight. After using this lens for a while, I find the Canon 24-70 mark I is annoyingly large and heavy. So was the new Tamron 24-70 stabilized lens that i rented. It's a nice lens if you need stabilization, but I felt that my old 28-75 did everything I need with substantially less bulk. Instead of buying the new Tamron, I bought a couple of the less expensive L primes and I'm very glad I did.
I got together with my local photo buddy today to help him confirm the MFA on his 7D with the Canon 50mm 1.4 (+5 MFA needed) I was surprised to learn that he has become very attached to his (much newer than mine) copy of the Tamron 28-75.