I think you're exactly the reason Kastle delivered the MX83 in lengths that are smack in the middle of those of the MX88. You're 'in between' lengths for the 88, which is quite common, so the 83 has been designed with you in mind. That being the case, if you buy the MX83 in 173cm not only do you get a fantastic ski that will handle virtually anything you throw it at, but it's been tailor made just for you. How nice is that?
If you read Dawg's reviews of the MX83 (see below) you'll see he's perfectly happy on that ski in light powder and crud. One thing he does mention is that he's quite sensitive to the length of ski. The 83 seems right for you on this front.
No doubt you've already found them, but here are the reviews. Dawg is a smaller guy than yourself (around 155lbs from memory) although I imagine he skis more aggressively and that would largely make up for the difference.
FYI, many people will own the 88 simply because this is the first year Kastle has released the 83. Previously the decision was between the MX78 (also a great ski - I own that one in 184cm) or the MX88. Or the MX70 or the FX84 or the FX94 or the BMX88 or the LX82 or the LX92. Sheesh!.
They're both great skis for your purpose, and you can hardly make a bad call.
Best of luck.