I have driven and fished out of a PHX 721, Z520, and a Skeeter FX21 (w/ the SHO). All are superb platforms with smart storage layouts and consoles. The Skeeter w/ the SHO was quick out of the hole but lagged a little in top end speed. Not an issue for short runs because the hole shot put you way ahead of everyone else. The PHX 721 was a little slower out of the hole but had good top end speed (72+ GPS). It handled magnificently, esp in ruff water. The 520 was heavy - slower out of the hole and at top end but very smooth, esp in choppy water.
I just bought a new sled this past May. I went thru every option out there, considering every detail and option. It got to a point where there was so much data I couldn't keep track of it. In the end, I picked the make and model that gave me the best collection of top end speed, hole shot, storage layout, and ride in ruff conditions (in that order). You can get to this point only by driving all of the options out there. Takes time but it'll help you make a [better] decision.
I stayed w/ the 2-stroke Pro XS. Yeah, at some point it'll no longer be in production but I'm not one to buy into something in its first model year. Merc has some nice ads out there but my gut (and experience) say go w/ what you know is ~bulletproof. If I were to buy a 4-stroke it would be the SHO. It's solid. Not the most fuel efficient but we're talking fractions of miles per gallon here.