Charles said
I have similar tastes in tone, and both of those are on my short list for consideration. There's a kicker, though - all of the Scott Caos are copies of various famous violins. So what you hear when you hear an SC750 depends partly on which particular one it is a copy of.

The ones that Fiddlerman has made videos of I tend to like quite well, with one that I wasn't as impressed with. Unfortunately, he doesn't say in the video (or in the text below), which specific violin the one he's playing is modeled after. I don't know whether the difference had more to do with what it was modeled after than the variance in individual violins or not.

On the Soloist, there were two example that I didn't care for so much, with all the others being quite good. Since (as far as I know), those are all the same instrument model and maker, there may be more regularity in what you get.

For my own personal ratings, the SC 750 scored a bit higher than the Soloist. So if someone forced me to make a choice today, between only those two, I'd probably go with the 750. Between the higher score and the lower price, I suspect I'd be happier with that.

However, FM has excellent customer service. If you ask them (him) to, he'll demo the violins he has on hand of whichever one you're most interested in. (Might even do both.) Ask him to identify which model the SC750s are (Cannon, Kreisler, Lord Wilton, Amati, whatever), if you go that route .  (For my sake, if you don't care. )

One other recommendation - have your sister listen to the Youtube videos of the demonstrations he's already done. (Get on Youtube and search for "fiddlershop scott cao 750" or "fiddlershop fiddlerman soloist".)  Her opinion of each is going to matter more than either yours or mine.  

Thank you for your advice. Yes, I pay a bit more attention to SC750 like you. But cannot deny FM SOloist is such a potential violin in future. 

My sister she has done some research online. she saw a violinist play on Jay Haide violin and she said that she couldn't stand falling for Jay Haide violin ( tone as well as its appearance) She told its tone is deep and soft and warm . I do know Jay Haide in range of $2400 violin and it is far of my budget right now. We're international student from Vietnam so pay that amount of money is too much for us even pay for SC750 or FM Soloist also is our years savings. I am just thinking of another way that we might upgrade later. Maybe to a Jay Haide violin. I wonder if Fiddlerman has any offers of Jay Haide in next few years or not. But If he does,I'll definitely buy it from him because I love his service so much

MrYikes said
Have you thought about a viola?  

 Hi,I love both and my sister just loves violin. Idk why