Personally, I think the 7300 and the 710 are pretty much the same thing, with both having their own advantages and disadvantages. If you're buying one, just focus on the price. Unless you have a specific need that requires one over the other. When the 7300 was first released, it also received a lot of criticism due to its quirks.
If I were choosing, I would select the Japanese version, model 7300.>Used unit, Model 710>Retail DX10
The Japanese version of the 7300 costs over 5,000. If you have a friend who can buy one for you, you can get it for 5,000. The price of a genuine 7300 is not worth considering.
For the retail version (710), the price during the promotion is around 6800, without speakers. With a 11.11 sale coupon, it's less than 6400. The price is not much higher than the Japanese version (710), and there's also a 3-year warranty.
Before the 710 was released, the price of used 7300s was quite high. After the 710 was released, the prices of used 7300s dropped significantly. Furthermore, issues with the exchange rate between the Japanese yen and other currencies also caused the price of the Japanese version of the 7300 to decrease even further, which had a significant impact on the market for used 7300s.