Picking a Niche + Keywords

I've been doing some niche research, and have found 2 different niches, lets call them Niche A and Niche B

Niche A:
-Is something I am really really interested and knowledgeable about
-Key words have a fair bit of competition
-The actual product has very little competition
-When suggested to the target market, they are interested in buying it, however, it wouldn't be something they would search themselves.
-Getting ranked for any key phrase would be quite difficult

Niche B:
-Is something I am sort of interested in, and reasonably knowledgeable about
-I've only managed to find 1 low competition key phrase, which also has high 'commercial intent'
-Might be hard to convert since there is other free material available
-Getting ranked might be easier than Niche A, but writing articles/tutorials would be a bit more difficult and not as enjoyable as Niche A

Which would be best for a newbie like me? A, B or should I keep searching and find something which is 'just right'?

One last question: When searching for a key phrase in quotes, what would be the best amount of results for easy ranking for a newbie?

Cheers,
Sam