I haven't fully refined my thoughts, and I'm going to be developing them as I'm writing this response, so let's see how it turns out.
The first thing that occurs to me is that context is everything. I don't have a single style of addressing this issue. It depends not only on the game system, but also on the campaign, and even who the other players are.
1. When I'm playing in a campaign or system that is very focused on
a theme I usually start with WYA. When something explicitly evokes that mode, I jump right into it. White Wolf's WoD games aim to do that (whether they succeed or not may be debatable).
2. If I'm playing in a game that is highly
crunchy in a
limiting manner (such as D&D), I tend to focus on WCYD first. I have to see what my options are so I don't
waste time thinking of a concept that cannot be represented in the system.
3. If I'm playing in a game that is highly crunchy in an
enabling manner I may focus either way. Context is the most important in determining my approach here. Is there a strongly presented theme? See #1. Is it a blank slate given to me with a ton of KeWl PoWerz? Then I might look and see what powers look the coolest.
4. If I'm playing in a game where I know the composition of
the group matters, then I will look at both WYA and WYCD based on what I know of the system. One thing I personally dislike is to come into a D&D campaign where the party doesn't jive and cover the bases, whether that is due to WYA or WCYD incompatibilities. While I'm a fan of D&D parties that are deficient when it is a theme (such as an all fighter party, all mage party, etc), but most D&D parties want some level of covering your bases in order to be most effective in a standard campaign. So I will make a character to cover whatever base is missing. This is rarely any sort of drag on me, since I most enjoy the immersive method acting play style and can enjoy role-playing a vast number of different characters.
5. If I'm playing in a group that
optimizes (assuming I would do that again...probably not) I probably have the most difficult time figuring it out. Optimization makes it difficult to support most realistic WYA concepts, because most real people aren't optimized. Since I hate the forced feeling of taking a pile of stats and deriving WYA afterwards, this is a struggle for me. Last time I ended up with a character concept I loved based on WYA who I had to do a heck of a lot of work on to attempt to get half-way optimized. I had an idea of a street urchin turned guild thief, turned conscientious guild deserter. His free-wheeling, wise-cracking, good-natured, "never take anything too seriously" attitude and style was a real delight to play. I got to express a part of my personality that I rarely do. He's still one of my favorite characters that I've ever played, and I wish I hadn't had to drop out of the campaign as soon as I did. But this was in Pathfinder. I multiclassed for concept, but my DM was concerned that I might lag too far behind at higher levels, and I ended up actually multiclassing carefully into 4 classes (by level 8) and really doing a feat hunt in order to try not to lag too far behind while still realizing his concept appropriately. His combat contribution was present, but not impressive, and his greatest contribution ended up being through his personality. The rest of the group seemed to really enjoy the character, but I had to fight with the rules to be a mechanically valued member. Fortunately I enjoyed gaming with the group and had no particular dislike of Pathfinder (especially at the time) which meant it was really fun anyway. But it wasn't easy to play WYA in a party where WCYD was important.
6. How
long is this campaign going to be? I'm not a fan of spending more time creating a character than I do playing it--although I have done that on more than one occasion.
Sometimes that is all thrown to the wind and I do something completely different. For instance, I have a character that first came into being back in AD&D and is my favorite character concept--the kind I tried to recreate in various media several time. His final incarnation is in an upcoming 5e mega-campaign. His initial concept was an idealized self-projection power-character idea created by a kid just learning D&D. Half-elf fighter/mage, fights with a longsword and wears purple. Over the years the concept has developed. I still can't entirely say his personality is fully developed (since it has changed over the years) but since it is partly an idealized self-projection, I can more or less go with "what would I do?" when in doubt. The most cheesy origin for a character, yet one that will always resonate with me.
In general, I'm not a fan of starting with WYCD and then attempting to derive a personality of the character. On the other hand, it is rare that I create a character based purely on WYA without an awareness of the system and variables of the group, campaign, etc. It's probably most accurate to say that I allow both WYA and WYCD to develop in an interrelated way in my mind based on both my interests and the campaign.