I really couldn't imagine anyone being disappointed if they did , the S54 ZMs are great cars but the whole package of 997 ownership is a step up & by some margin , i'm not in any way rubbishing the ///Ms but i can't think of one department any of the 3 ZMs ive owned would get the tick over the 997

( granted original list price not comparable but used values now are not that far apart )
As for the question you asked ? Obviously costs of service are dependent on small indy / large indy / main agent & gen1 cars are slightly less than gen2 on certain services ( plugs being harder to get at on the DFI 2s so more labour costs ) but taken as a direct comparison over 2yr-4yr-6yr-8yr-10yr-12yr id estimate the 997 would be cheaper to follow the advised factory regime .
Note the 12yr marker is deemed the MAJOR in 997 servicing , for a idea on that its around £1200 at a main dealer / £800 decent indy but covers a few more items than a S54 Ins2 ( 4wd box fluid on the C4 / serpentine drive belt / brake fluid / coolant / new tyre gunk )
For a grasp on a range of costs at a reputable indy have a sift through the menu at RPM Technik https://rpmtechnik.co.uk/servicing/fixed-price/porsche-997-c4/