I have
a) lived in India/Delhi as a single woman for 18 months about 4 years ago
b) travelled to Nepal while I was there
In my experience, the two are not comparable. I loved aspects of both but Nepal is a totally different environment to India. That may well be because my experience of one is living there but my experience of the other is as a tourist, but I found Nepal to be a more relaxed and comfortable place to be.
There was much about India that I loved, but some aspects that I didn't:
I was a target for everyone who needed money. I understand why, my own wealth must seem astronomical to some people (though it isn't in the UK) but the impact of that is that, everywhere I went, I could see people may a bee-line for me to sell something, offer something etc. And the price is always inflated (something an Indian friend of mine called "skin price"). If you are not comfortable with saying no or haggling hard, you will be parted with your money very quickly.
I was often a source of curiosity. I am white, blond, blue eyed and was often alone. I was stared at everywhere I went and, in many places I was approached - often for nothing more sinister than strangers wanting to take pictures of me, or with me. But again, if this makes you uncomfortable then it can be unpleasant .
There are considerable restrictions to your freedoms because of safety concerns. You cannot just jump in any old taxi at the end of a night. You cannot walk alone in many places (especially at night) and you need to be vigilent during the day. Which ends up feeling a bit 'guilded cage' after a while.
The Empire and it's influence on India is still apparent everywhere, and not in a great way. I often found myself reminded of the comedy sketch with Mitchell and Webb in which they, dressed as German soldiers, started to question if they were the BAD guys after all. This is both good and bad: it is uncomfortable but it opens your eyes to how the world may view ritain and why.
All that said:
There are some stunning things to see in India. Kerala was beautiful, the Taj Mahal is genuinely stunning, the Golden Temple equally so, the Wagah-Attari border ceremony is a great spectacle.
Lots of people - like anywhere - are genuine and friendly and I have rarely laughed so much and so hard as in India, both with friends and strangers.
I agree with others, you want to book everything (travel, accomodation, even eating to some extent) with an agency that you 100% can trust. It will make all the difference.