I've had 8 ZTs and sold off 4. I've had 9 Shiro's, kept 8, and only sold the one to fund another. My 4 ZTs are an 0452CF, an 0450 modded with a CF scale to match its big brother, and 2 0801s--a BW and one ano'd and pimped. My take on ZTs is that they mostly flip aggressively due to stiff detents. I like my two Sinkevich's but my 0452 caused my forefinger great distress for a long time until it finally broke in. The two Rexfords are more my type of flipper--certain, but more subtle, their flipping action more a product of good design than restraint by an overly tight detent.
That's how I see my Shiros. They flip quickly, but easily, coming out certainly and with sufficient authority but not trying to jump out of your hand either. My measure of a good flipper is how lightly I can depress that tab and get a sure opening and my Shirogorovs uphold that standard better than any other knives I've handled.
The other thing I look for in frame and liner lock knives is ease of disengagement. Some of my ZTs gave me fits unlocking, but I have considerable arthritis in my hands from almost 50 years of carpentry work. No problem unlocking my Shiros--with easy access and disengagement, they don't hang up on the detent ball, and they either free-drop or close with a light shake depending on how I have them tuned. Mostly I index finger flick them closed, though.
I don't know that you can quantify in dollars how much better one knife is than another. Various knives cost what they cost and it's up to you whether it's worth it or not. Given the choice, I'd rather have one Shiro than 3 ZTs, but if I just wanted to try one out I'd think about selling one knife and saving up the rest of what it took to take advantage of a good deal on the Exchange.