Originally Posted by BubbaX

You can always pay the change fee on your ticket and buy a new one in the higher class. You can also try calling Upgrades become available at online check-in by selecting a seat in the desired cabin, although if not immediately available, they might be visible at 24 hours before departure.
You can inquire when checking in/dropping bags at the airport, and, in the US, at the gate. Finally, if you spot an open seat on board, you can ask the purser about buying an upgrade, although you may not get the catering.
The AF W shell seats are not great, and are genuinely garbage for sleeping. You still get a little more personal space. The first meal in Y/W in AF is generally edible, and, as noted, W gets the same main course with a slightly upgraded side and silverware. The second meal is insulting garbage.
KLM's catering has traditionally been less impressive, but in Y, I imagine the only noticeable difference will be the lack of champagne. In W, KL could very well be better.
Now, in J, AF's seats are generally better than KL's; the KL 777s generally have a 2-2-2 layout up front, but beware the dreaded NAV4 layout on AF. And food/wine on AF beats KL. Cabin crew: generally, people find it easier to get along with KL crews, but I generally have fun with both.
Catering from North America back to Europe will be substantially the same on both airlines (except for the wine).

​​​​​​in other words, it's all a matter of where you want to go and what you want to pay. Personally, I would put a lot more weight on the schedules, prices, and relative importance of layover location.
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