MacBook Pro OS X El Capitan clone to my MacBook air?
I maintain a similar setup, with a mid-2010 MBP on the home desk (using an external monitor, keyboard, and mouse) and a mid-2013 MBA for travel. To maintain a consistent user environment, both machines run the same macOS, currently Sierra 10.12.6.

Contacts, Safari bookmarks, Calendar, Keychain, and Find My Mac are synced using iCloud; I use no other iCloud service. My email is IMAP, and thus synched by my ISP's server. 

The two computers are synched over an Ethernet connection using ChronoSync (on the MBP) and ChronoAgent (on the MBA). This synchs the computer's Desktop folders, my Documents folder, and Photos. I do this just before travel, and just after return. After the first synch, synching usually takes about two minutes. 

I found ChronoSync clumsy to setup, but it does the job. I also use CS to each night create a bootable backup of my MBP's SSD boot drive.