1) Which one is better ? And newer ?
Regarding to SSD, it doesn't about the newer the better, you need to look at the specification. To me, the difference of both drive is X300s got the self-encryption, so the writing speed would be slower (correct me if I am wrong). When you compare the speed of both drive, I would say Sandisk SSD Plus is better because its writing speed is better the X300s.
2) If I connect them to pc the one with 240GB show only 223GB and the one with 256GB Show me 238GB It’s a 15GB Difference in Capacity storage IN THIS CASE
What you recommended ?
As I mentioned before, it depends on how often you use the encryption. If no, then choose the regular one (SSD plus). The discrepancy of disk size is the over-provision. SSD allocated space as buffer and data correction. It doesn't matter about the amount of size.
3) they both sata III (3) ? if no , its Critical difference between sata 3 and sata 2 ???
Both of them are SATA 3, the difference of Sata 2 and 3 would be the speed, the estimated speed of SATA3 is 600MB/s which basically no SSD can do that. Plus, if your SSD got the 5XX MB/s and it is considered as high-quality.
4) I plan to used the ssd drive with adapter cable usb Directly to computer like an External Hardisk to backup some files and stuff it a good idea ?
if you use them as external hard disk, I would definitely recommend to get a regular HDD is fine. (personally suggested TOSHIBA, at least I didn't broke it so far )
USB adapter connection speed will not fall down? There is no such thing about not dropping connection speed and it depends on your USB cable and the read/write speed of your disk.
5) Maybe After a while I'll use it to connect internal hard drive for the operation system so which one better for two cases
If you are concern about the writing speed, use SSD plus then. I hope these can answer your question