Both of those prices seem really high.... '14 Carbon Spyder should sell between $350-450 depending on condition. RX3 I think we've already talked about but I can't see spending more than $800-900 for a used for a RX-3. Bare bow pricing, anyway.
And, to answer the actual question

, I've had both, along with a bunch of other Hoyt's. The Carbon Spyder was just a special bow. It has great cams, they overachieve, it tunes easy, and is of the carbon era where the bare bow weight is advertised correctly

. The '14 also has the pre-ZT cable guard which is preferred by most over the ZT. The only drawback of the Spyder line was you have to pay attention to the strings, they can wear excessively on some due to the string rubbing against a post on the cam. It will be obvious if you look a the string near the cam, there's a few threads on it as well.
The RX-3 was a similar breakthrough - great cams, maybe not overachievers like the CST but they met IBO and pulled very smooth. Carbon is a bit heavier than advertised, and they did have the sight plate debacle. But it's still a fantastic bow, and probably Hoyt's best since the Spyder/Nitrum lineup. The RX-4 makes minor improvements on the RX-3 making it even better.
Personally I would (and did) choose the RX-3 over the Carbon Spyder Turbo. But the CST is still a great bow at normal pricing ie around $400. Factor in you prob would need to replace strings if not already done so add another $80-100. At $675 I would pass though, they pop up in the classifieds and on ebay pretty regular for less.