hi Roberto,
There isn't really any "versus" in whether you should go N7K compared to Catalyst 6500 - both are products with a long lifetime ahead of them.
Certainly, we have seen numerous people choose to collapse the core and agg/dist layers into a single layer - that is a valid design - but like all design decisions, whether its appropriate for your environment depends exactly on what your environment is today, what it will be in future, and how you expect to transition between the two.
Things to consider here is: transition from gigabit to 10 gigabit, unified I/O, what server virtualization may be happening in your environment, etc.
Another thing to consider is the structure of (existing) physical cabling - i.e. if you're reducing the # of devices in the network, is the structured cabling set up in such a manner that you can do this easily or not?
Yet another consideration is exactly what features and functionality you are using on your Catalyst 6500s today. Nexus 7000 and NX-OS doesn't have the same sets of interfaces or breadth of features/functionality of IOS - intentionally so - but one would need to make sure that as a minimum it can do what you want today.
There is certainly lots of innovation with both products, an informed discussion with a technical Cisco person is perhaps the best path forward here to go through pros/cons.
Not sure if you are talking to someone technical at Cisco about all of this, but I'm happy to help - either directly - or by putting you in contact with someone technical closer to you to discuss.
If you want to take me up on this, email me at ltd@cisco.com
Regarding the last point about how cable management is done, if you do a search on "Installing a Cisco Nexus 7018 Chassis" on cisco.com, you'll get a link to http://www.cisco.com/en/US/docs/switches/datacenter/hw/nexus7000/installation/guide/n7k_installation_7018.html which does show the cable management on an 18 slot chassis. There are quite a few incremental improvements over what you have today.
cheers.
