I have WH2O and I have researched extensively about the Shadow, I plan to have it as well. If possible, get both. Both player games, vastly different.

As for maintenance WH2O's burden is:
- many ramps need much $$$ for keeping pin in immaculate state. Bigfoot ramp gets busted without plastic protector, Whirlpool ramp gets cracked in a part sticking into the playfield, both left and right ramps get cracks in lower part where the ball bangs just before dropping into inlanes, subway ramp gets very dirty very soon. Except for bigfoot ramp everything is purely cosmetic, depends on your OCD level. If you need your ramps mint and clear, it would cost you dearly to replace them.
- bumper caps -> "boulder garden" get broken also. One online retailer has individual plastics, others sell all plastic mountains as a $129+ kit.
- upper playfield gets divot for a ball to sit in at the VUK. You'd need relatively inexpensive cliffy protector.
- one place at left slingshot always gets a wear spot all the way to the wood. Inevitable. There is a overlay for it.
- on my WH2O VUK ball cup falls off regularly. A nightmare to screw back because it is heavily obstructed by a large number of ramps and a wireform. Maybe the screw is too short. Currently I leave it without a ball cup.
- playfield tear down is a nightmare due to huge number of elements.
- upper playfield often develop planking

The Shadow has a lot of things to shoot for and IMO complements Whitewater nicely:
- It has 6 modes when you want variety. Whitewater is not a mode based game. Both have goals to progress to the end.
- It has a wizard mode with 5 ball multiball. This wizard mode is goal based, you have to trip each switch. No wizard mode in WH2O and only 2 and 3 ball multiball, but there is a cool sort of wizard (although it isn't really) award that is very hard to attain.
- It has unique mini game in a form of a upper playfield and a goal to defeat it to progress to wizard mode.
- It has a very nice Jackpot and Super Jackpot system for multiballs. More complicated than WH2O which is also nice in this department.
- It has Mongol and Vengeance games to add variety when you don't want to play regular 6 story modes. WH2O only has Bigfoot hotfoot hurry mode and Man Overboard hurry micro-mode.
- Shadow has no bumpers. Whitewater has 3 and they are covered in expensive plastics.
- It has a great and very difficult video mode. Whitewater doesn't have any.
- Both games have combos.
- Both games require precise shooting and punish mistakes.
- Both games have flow.
- It has two ramps (not plastic) with player controlled diverters for ramp shoot variety. Still Whitewater has more variety in ramps and passages.
- It has a nice magnet controlled lock with no play value other than awe from audience.
- Both games have nice DMD animations and easter eggs within.
- Both are 3 flipper games.
- The Shadow has an automatic plunger with STTNG type plunge award that still require player to shoot a ramp upon plunge. WH2O has a manual plunger with skill shot requiring to drop a ball on the upper playfield (via steep plunger wireframe) and hit a ramp with third flipper. IMHO, much more variety with TS.
- WH2O has a topper that is very costly.
- The Shadow has better, DCS, sound.
- WH2O has a way better translite.

Regarding TS maintenance I read that:

- upper playfield may cause mechanical problems
- replacement decorative rings on ramps are hard to find
- replacement mongol figurines (3 of them) are hard to find
- replacement complete plastics set very hard to find
- replacement diverters (Phurbas) hard to find
- magnet burn on Sanctuary and divot common problem equivalent to Whitewater lost mine wear and VUK divot

I hope that my post was helpful if not original enough