DsMo of Chei is definitely my preference over DsTm, although the second is reasonable, too. Let me come back to that later.
If by midgame you've kinda won already, please abandon at this point your god and continue as atheist
If you worship Xom, you might not notice quality of good god in late game, but other people usually benefit from their gods the whole game.
Even if you get superb utility spells early, you rather shouldn't train them immediately, because jack-of-all-trade simply won't work well when you meet first hill giant. +15 int makes later spell learning very easy regardless whether you learn only repel/blink/shroud or go towards deflect/invisibility/darkness. Of course spellcasters make greater use of int than fighters, but trying to prove how +15 str makes your mage superior is even worse. Overall I think that melee-oriented hybrids make the best use of all 45 points in total and feel free to disagree, but don't minimize pros of good int on monks.
Let me also raise the issue of Ds aptitudes. With slightly more expensive skills than usual, you really do not want to waste any exp. Transmuter needs quite many skills to develop and it is hard to get quickly all of them. Not being able to outrun enemies in spider form is serious drawback on this background. That is, Ds isn't as good Chei transmuter as a species with natural transmuting talents. Also transmuter somewhat loses benefits of being demonspawn, because blades can replace claws and ice form/ spider form will replace any body mutation.
The exp issue hurts mages even more if they want to make more serious use of additional strength in melee - and they are kind of forced to, because sooner or later they will find themselves out of mana. Further, lack of guaranteed spells always discourages me even more from trying such combinations.
Getting piety is easy, but bonus piety on Chei has very nice advantage. If you meet ogre soon after you've converted, you won't be able to outrun it with 0* piety, but you'll have significantly greater chances to kill the same ogre with 2* piety thanks to already somewhat boosted statistics and access to first power. Also having earlier access to chei's second power rises your survival chances against e.g. centaurs.
I'm not going to argue with any formulas, although I don't trust their outcome. Fsim shows in most cases much more than 20% boost (I tested for 15dex, 15str vs. 30dex, 30str). Also my experience says that 20% isn't the case!
Chei is a god you need to learn, seriously. He's not obvious, doesn't forgive errors, requires a lot of attention and planning. If you do not have patience, just look for something "easier" to play. It doesn't mean that Chei games are that difficult - they are in fact relatively easy, but I doubt you can figure it out in 2 hours.