Which is more common in Cantonese? Do more people pronounce 你  as leih or neih?
In most textbooks, and practice dialogues I seem to see it is 'neih'
For some reason, leih comes more naturally to me, and I am feeling like I heard it more publicly.

Does it have to do with region?
As Stumpy Joe says, N and L have merged. Even outside HK it is quite generalized now -- to the point that people below 40 have difficulties -- up to complete inability -- pronouncing initial N-. I don't know how many times I've heard "What is your lame?" or similar stuff...

I work in the wine business, and every time we have to translate a wine name in Chinese for the Chinese back label, if there's a syllable with an N or an L, it is a complete crapshoot. N- syllables sinicized as 利, L- syllables rendered as 呢, etc. When I point out the errors, my Guangdong clients, even the older loubaan, seem to completely miss the issue: 一樣㗎! Not quite, brother. On the other hand, wines imported in the north usually get named with Ns and Ls in their proper places.

For instance, the famous vineyard Pont-Canet is usually transcribed 朋特卡內 (it's imported by just about every company in China, so a Mandarin sinicization prevailed), whereas the excellent La Tour Carnet vineyard, whose wines arrived in China mostly via the south, was named 拉图嘉利. I tried to get that changed to 呢 or similar -- my clients couldn't see what I was talking about. For both 嘉利 and 嘉呢 my clients said gaa.Le(i).

On the other hand, some of my friends, who speak average to good English, are vaguely aware of the L/N issue, but can't control it 100%. N- appearing in L- words happen regularly. A friend of mine is very fond of saying nousy for lousy. I tried to tell him it's Lousy. He said, Yeah, nousy!. Hopeless...