For what it's worth, I would stick to "benignity", only because the former is more common in UK English, you only need to do a search on google to verify this.

Anyway, here are my findings:
http://66.102.9.104/search?q=cache:t3nPKYfNGZoJ:www.answers....


Library > Words > WordNet Synsets
benignity, benignancy, graciousness
the quality of being kind and gentle


benignity definition
n. pl. be·nig·ni·ties
1. The quality or condition of being kind and gentle.
2. A kindly or gracious act.
benignity synonyms
noun
1. Kindly, charitable interest in others: altruism, beneficence, benevolence, benignancy, charitableness, charity, goodwill, grace, kindheartedness, kindliness, kindness, philanthropy. See attitude, kind
2. A charitable deed: benefaction, beneficence, benevolence, favor, kindliness, kindness, oblation, office (often used in plural), philanthropy. See give, kind


benign prostatic hyperplasia definition
n.
A nonmalignant enlargement of the prostate gland commonly occurring in men after the age of 50, and sometimes leading to compression of the urethra and obstruction of the flow of urine.


Collins EN-EN
Benignant adj. 1. Kind, gracious. 2. A less common word for benign (senses 3, 4) – benignancy n.

Benignity n. pl. – ties. 1. The quality of being benign 2. A kind or gracious act

CancerLIT: 704520
Differentiation of malignancy from benignity on the basis of early and ... spindle cell histology in 13 patients and epithelioid histology in four patients. ...
cancerweb.ncl.ac.uk/cancernet/cancerlit/704520.html - 82k - Cached - Similar pages

PDF]
PROCEEDINGS AND REPORTS OF UNIVERSITIES, COLLEGES, COUNCILS ...
File Format: PDF/Adobe Acrobat
Benignancy. and. Malignancy. in Cartilaginous. Tumours ... histology. of. implanted. respiring. and. non-respiring. grafts. ...
www.jbjs.org.uk/cgi/reprint/35-B/1/144.pdf - Similar pages


Sure, they are both used in UK English, but the former is more frequent, from what I understand and see.