They are very similar. I bought in Lower Westville last year (between Fountain & Chapel). I have family in Spring Glen, and spend a lot of time there for my young kid to see his cousins. They both have the same feel. Friendly neighbors/community/kids playing & riding bikes. The houses are close together & typically older. Neither is bad choice, and I my full response would be paragraphs long. Feel free to PM with questions, or come for a stroll around the neighborhood.

SPRING GLEN - will be easier to Buy, more inventory and slightly higher purchasing power. Best Video is there

WESTVILLE - much more practical commute to downtown, and more going on (Yale Bowl, Westville Music, restaurants/bars in the village)

For me personally, we did not have a DEADLINE to move and told our realtor "we would rather focus on Westville and hope the right house becomes available. If not, we know we would also be happy in Spring Glen"

Anecdotally, my wife is affiliated with the hospital and 2 of her co-workers recently moved (sight unseen) from out of state. Both ended up in Spring Glen because their realtors said "you do NOT want to live in New Haven!"....they both said they wished they knew this was an option when they drove down my street.