Personally, I think the Sum to 10 method is the best for balance and the easiest way to build a Missions character. Mostly because once you've assigned your priorities, everything else just follows the normal flow. No chance of things being broken or unbalanced that way. Someone might have two A priorities, but they're going to pay for it when they have a couple Ds and an E.
Sum-2-10 is not the most balanced as 8-bit said, Karma is.
But, Karma is much more complex & harder for a guest GM/Character to verify is accurate which is pretty important in Missions I'm thinking.
That said, Sum-2-10 is my Favorite as its very simple, just like Priority, but allows you to have Multiples of the same "Letter/Number"
Very important IMHO for when your trying to say, make a Troll w/ Edge that also has a lot of Cash, or attr, or Skills, or Magic.
Also useful at the other end of the spectrum where your trying to make a character that is "normal" in his Cash (D) but has a little Magic (D) as well, a guy just awakening, gifted in Attr/Skills but not Rich or Lucky.
My only problem with Sum-2-10 is not with Sum-2-10 at all. Its with the Priority Chart. I
REALLY feel they should be equal jumps going from E to A and between the columns at least try to have a similar karma expense point.
But, really, even min/maxing your choices isn't saving you too much. I looked at Karma & Sum methods already & both weren't
THAT different from Priority. I think I ended up 20+ Karma better by tweaking Sum-2-10 & around 100 Karma worse via Karma Build to create basically the same character. Changing a dice pool by 1 isn't a make it or break it difference that would make me decline that option.