WHM/SCH is better, but WHM/RDM is very competitive depending on the situation due to a slightly increased spell list. Both are the best subs for White Mages standing in the back.

There are many reasons, but basically, /SCH is better because Light Arts makes everything better, and Sublimation replaces Refresh in situations where you don't get a Refresh II. With Light Arts you get cheaper spells, a better recast, your enhancing magic is better (C+ -> B+, basically your bar/boost spells get a massive buff, and you don't even have to cap your Enhancing to get the benefit of capped Enhancing), and you can pop a stratagem if things go bad. If it wasn't for Convert, /RDM wouldn't even be in the same tier as /SCH, but it is still the inferior sub (/RDM is a good sub but just simply doesn't match up, /RDM is also a good to great sub for many other mage classes too though).

Subs like WHM/BLM aren't in the same tier, and should only really be used if you have a niche reason, like Stun. Warp doesn't count unless 90% of what you are doing is traveling, and if that's the reason people ask you to /BLM, consider blacklisting them.

For those that like to think outside the box, WHM/NIN and WHM/DNC are full of win.