Whilst I can't pretend to make an income from stock, I have images accepted by stock libraries taken with both transparencies and negatives. Where I shop (live in UK, and usually buy film online from 7dayshop) the range of prices for slide and negative 120 film is roughly the same - both are in the £3.50 - £4.50 a roll range. I've processed both at home and don't find one any more difficult than the other (the main problem being avoiding dust and scratches which is common to both types).
Recently I have shifted from predominantly using Fuji slide films to predominantly using Kodak Portra negative film, largely because of the very wide exposure latitude. I have used Fuji Reala and like it a lot but I'm trying to concentrate on just one film type to reduce variability.
Portra is available in 35mm, 120, and large format, in speeds of 160, 400, and 800 - whereas Reala is limited to one film speed and to 355m and 120 sizes.