Have been asked this question many times as I assume both these amps are available on the preused market currently.
The 310 is a low powered integrated amp, commonly referred to as the "baby NAD". It is also known as the amp for the audiophile on a budget. Its rated around 20watts per channel but can produce some bigger sounds with the right speakers. However, since NAD kept it basic, it lacks a phonostage, hence you'll have to use an external one. My opinion - nice for a CD or Tuner based bedroom setup or ok for book-shelf speakers for tiny living rooms.
The 705 on the other hand is a stereo receiver, ie it has an inbuilt tuner. However i must say that this is one from the early 1990s. It is rated at 40 watts with an 8ohms load and comes with an inbuilt phonostage. The tuner is alleged to have the famous NAD 402 circuit and the amplifier section from the NAD 304 integrated amp. Given the fact that the 304 had a great phonostage, this amp is also likely to have a good one, which is the trademark of all older NAD integrateds.
Personally, if you ask me to make a choice between the 2, I would go for the 705 as it is a good integrated with an inbuilt Tuner and phonostage. However, I suggest an audition before making any audio-related purchase decision.
Pricing is very subjective depending on a lot of criteria such as overall cosmetic condition and functionality, previous ownership history, service history, whether being purchased in person on cash payment or via shipping, etc. Without info on all these aspects, I would say that vintage low to medium power NAD integrateds are usually priced in the 6k-12k range depending on these factors. The NADs that are currently in production or just out of production are priced higher. NADs purchased New from Indian dealers are usually priced heavily in the Indian pre-used market.
