I have both SX and Squire sitting within 3 feet of me. I like them both, and they are both different animals. But both good. As I have mentioned before I prefer the Fender/Squier neck radius of 9.5 inches over the 12 inch on the SX.
Other than that they are kind of a horse a piece. I like the SX much more as a modding platform due to the low entry price. I have 2 SX Jazz basses that will have a total cost in the 300 - 400 dollar range when they are both done with their mods. They feel like and have the same quality components of basses that are worth 3x as much as I have into them.
I have both a VM and a CV Jazz. The VM is stock and will probably stay that way. No reason to mess with it, it is just too good. My CV is heavily modded and about to get another big mod, so to compare it to a stock model is not fair.
However, I have now played well over 8 different CV Jazz basses and 5 or 6 P's in shops. I have never found a CV I did not immediately like. All sound and feel great.
Now all the modding aside, the SX is probably the best value out there. The Squier, especially the VM and CV are just flat out fantastic basses, and the price aint bad.
Despite the work I have done and am doing to my basses, both are quality instruments and neither feels cheap to me. My fretless SX is bone stock and may stay that way.
Also, with an SX it is reasonable to expect to do a set up and do not be shocked if a fret needs some attention. But for 100-150 dollars can you complain? I went into my first SX purchase fully prepared to have some fret work done. None of them needed it, though I had two of them done.
They are both now sold. Kind of odd really. The two where the necks had a fret job were the two I ended up not playing enough. So I sold them. Nothing wrong with either bass and both neck are fantastic. I just didn't play them as my VM is getting the lions share of my attention. The CV and my Sx basses are played frequently too.
If you worry about not liking a bass you have never played, or you worry about having to have fretwork done, and money is not an issue, go for the CV or VM. Play as many as you can. When you play "the one" you will know. Buy it on the spot!