The + on Flatron refers to the built in USB ports.
While all three are excellent monitors the Sony and Flatron are better than the Samsung due to dot pitch and refresh rate. The choice then comes down to Aperture Grill or Slot Mask. Some people complain about the support wires on the Aperture Grill being visible under white background conditions. I for one find them distracting. Bottom line the Flatron wins.
Sony is generally very good. Regardless of identical crt technologies, electronics make a difference. If price is no object get the gdm f520, it arguably the best of the best (certainly in price!). Due to the lack of good monitor reviews, I'd get a trusted name brand. Picture quality is the most important criteria given the relative closeness of specs. Don't always trust dot pitch, use it only on a very gross level (i.e. .22 vs .30). 75hz may be pushing the minimum flicker requirement (even if you can't see it may affect you subconsciously--I know it sounds funny but it is true--your peripheral vision is more sensitive to flicker--more light cones while color cones are concentrated in the center). I don't like sumsung except for flat panels (very highly rated in this category). Haven't really heard of flatron.
Quality is better than name brand, even regarding beloved AMD.
I just got the LG 795+, similar to the 915+ you mentioned and the 995+. It is very nice and you would have to compare it side to side with the sony to see any difference in picture quality. Except for the lack of support wires that is. The totally flat screen even looks slightly concave, just like the sony. How they manage that on a shadow mask is a mystery to me.
Given the high price of the sony (almost double), my choice was easy. But if money is no object (really?) then I would go for sony.
BTW samsung is discontinuing their NF line, I couldnt find one to test in stores.