It's hard to advise others on what they ought to carry as I don't know them, don't know what their goals are or what they like to do in their photography. But I can say what I'd do.
What I want for travel is light weight and compactness. What I've been reaching for through all the different kits and systems the past decade is to achieve what I once had with 35mm film cameras in digital cameras.
My last three-week travel trip, I carried the Ricoh GXR-M and Color-Skopar 21mm f/4, Nokton 40mm f/1.4, M-Rokkor 90mm f/4 lenses. It was a near perfect kit ... everything (including the iPad 2 and all accessories) fit comfortably into a small satchel bag (Black Label Bag "Oskar's One Day Bag Mark II" or A&A ACAM 7100) that weighed less than 7 lbs. That bag was convenient enough to *never* be left behind, although sometimes I'd leave the iPad behind when I was out on a shooting session.
Were I to leave tomorrow, I'd stick the same lenses and other stuff in the bag and then toss a coin as to whether to put in the M9 or GXR-M body. With the M9, I like the wider FoV of the same three lenses.
Having once traveled with a full 35mm SLR kit AND a full Hasselblad kit "for fun", I swore after getting home that I'd never do that again unless someone was paying me for the burden.
My rule of thumb: One bag. If the bag with everything in it weighs over 8 lbs, I've got too much stuff. The only acceptable addition is a light tripod.
It goes without saying...
if my thing was doing wildlife photography and my "vacation" was a safari on the African savannah, the above kit would be mostly useless so of course I'd change gears. And hire someone to schlep it all for me ...
=== Oh yeah === backup ...
Funny, but in 40+ years of photography, I've only had one camera fail and that was my own damn fault. For backup, I plan to either borrow or buy something in the places I like to go if the camera fails. After all, if I'm not getting paid and I'm on vacation, I can just enjoy the trip and forget the bloody photography if the camera craps out. And I'm sure I'll have my iPhone with me anyway for the memories in happy snaps. There are more important things in life than making every possible photograph I can.