I was in Da lat and Nha Trang two weeks ago. Although the distance is short (in kilometers rather than traveling time) there is world of difference. In Nha Trang there are Russian and Chinese tourist everywhere! (almost every sign is in Russian) - I will be polite and refrain from commenting on Russian and Chinese tourist. It was soaring hot in Nha Trang but of course the beach provided some relief. Parts of Nha Trang turns into a party zone at night with several beach clubs - I was at Sailing Club which was a great venue with Dj etc (I had a great night).
Da lat on the other hand is in the mountains (around 1,500 meter) is which cooler and very pleasant. Da lat city is quite charming (but very quiet compared to Nha Trang) and easy to get around (I rented a motorbike) . To put it in short: I thoroughly enjoyed Da lat whereas I regret I spent 2 nights in Nha Trang.