How do I choose between two girls: one is religious and I've known her for years, but she is still a student; and the other is the neighbor's daughter, whose circumstances and religiosity I am unaware of, coupled with the fear of problems that might arise due to the neighborly relationship?

I advise you to supplicate frequently to Allah to facilitate for you a righteous wife, for the Lord is near and answers prayers. The first girl is more preferable, due to what you have learned about her religiosity and goodness. Her studies might not prevent her marriage. Seek Allah's guidance (perform Istikhara) regarding the matter. If the marriage does not materialize, then you can propose to the other girl after inquiring about her and verifying her religiosity and character, and seek Allah's guidance (perform Istikhara) concerning her.