It depends, as if my cosplay group or boyfriend ask me to be a character for a group or couple cosplay. Personaly i chose the character, but with Panty (transformation, school + red dress) it was the costume as if im honest i suit Stocking more personailty wise, but i desided to be panty so one of my friends could be stocking as they perfured that character. XD

Guess im a bit of both :3

Character always.

Looks at all my lame arse easy costumes... I swear Its not what it looks like.

Really it is for characters I love. Someone could point out a really awesome looking outfit knowing that I cosplay. Then they go to subtley encourage me to do it. Meanwhile I'm sitting there thinking... Yeah he looks great. Don't really wanna be seen as him though...

I have a long standing bet that if I make a Wolverine cosplay before I'm 20 i shall win £200... I don't think I'm gonna win that. It's just not a charecter I think I wanna do. Damn i'm gonna lose £200.

There :3 told you its not what it looks like.

I read/watch/play something, and I end up liking a character. I then, inevitably, end up cosplaying that character because I cannot resist. It's the character themselves that makes me want to cosplay, but I guess that includes both personality and what they look like. I'm not known for doing a generic type, nor costumes that suit me. Seriously, I don't think I particularly suit any character I've done before. Anyway, I would never do a cosplay just because I like the look of it. Personally, I would have a real issue with posing for photos and for having a bit of fun IC. Oh, how I love being IC. Even if it's just for a short time. XD

Its all about the character with me. If its something I can relate to or the personality of the character i admire clicks with me then its all good...

.... until I sometimes see the costume say to myself ABORT! ABORT! but if doable then all systems go baby!

Like numerous people have stated before being in character is so much more fun. To basically be the character makes the experience more memorable to me. Esp when confronted by a character whom your character can related to or has connection with. Its fun to try out characters with vaired personalities. Franky from One Piece being really eccentic energetic and very physical to when I did Heroman whom was a silent giant robot. With him it was fun standing there watching people walk by and seeing the kids walk over and get a closer look as this robot who cannot talk is emotionless but they were fascinated. So yer character basically. THE CHARACTER BRINGS THE FUN!

if all goes well watch out for me being the Hulk at Expo

For me it's both.

I've got to like both the character and the costume or else it doesn't get done.

But what makes me choose a costume in the first place can vary. Sometimes it'll be because I spot a costume I like and then I'll get to know the character. But other times it'll be the character I like and so I pretty much go hunting through their wardrobe to see if there's anything they wear that I like

For me its mainly the character but occasionally I have gone for the costume. Some times I have had costume suggestions made to me where I'm not familiar with the character at all (like Sir Integra from Hellsing) but I have liked the design so I do look into it.

I do much prefer to like the character I'm doing through so I won't start a costume unless I have researched into a character.

I choose to cosplay because I love the character and find the costumes are interesting

That kind of depends
My first self-made cosplay was Matryoshka Gumi from Vocaloid, I only really chose it because it was simple to make
And when I do Vocaloid cosplay, I usually tend to cosplay different versions of Miku even though she isn't my favourite Vocaloid because she tends to wear teal/turquoise/green etc. and that's my favourite colour
But normally I tend to cosplay as characters because I love them or the series they are from