This is a very good question and I hope to shed some light on which lens to pick when shopping, or help you decide, as I had to, which lens to sell. This is primarily your decision but there are a number of factors that are outside your personal shooting style I think everyone should take into account if making this choice.
This article is intended for users with crop sensor DLSRs. For example Nikons D90, D7000, D5100, D3200, etc. Canons lineup includes the 7D, 50D, EOS 550D and 650D. On these bodies the approimate field of view for a 50mm is 85mm and the 35mm is 50mm (80 and 56mm respectively).
The decision of which lens to keep if you have both is probably easier to answer than picking the lens to buy. Obviously you can easily go back through your photos and count how many are on one lens vs the other and this method is suggested as a way of picking quite often online. This can be done via EXIF info. Picking your most used between the 50mm and 35mm would be an easy way to do this and many sources online suggest doing it this way. The result will be your most used lens, not necessarily your favorite.
In my experience when travelling with both a 35 and 50mm I used whatever was on the camera. It did not matter which lens was mounted and for most things either would suffice. Obviously there are some cases where a 50mm is too long and you cannot get a shot because there is no more room behind you. Likewise needing more reach with a 35mm.
So how do you decide between these two lenses?
I would look at the quality of the two you have on offer – for instance I picked between the 35mm dx 1.8 and 50mm 1.8 “nifty fifty”. I took the 50mm because I did not like the bokeh/out of focus on the 35. I also like the background separation on when using a longer lens.
To top it off my plan was to go full frame in the future, and the 35 DX is not overly great there. Picking a lens that suits what you like to shoot and enjoy taking photographs with is more important than counting photos and comparing specifications.