both are thoroughbreds that get out of the way of the music and allow a system to really shine. and lifetime keepers.
personally i would prefer the 458 mono blocks. the 458's will have a slightly better, more special first watt, and will handle musical flow, authority, space and scale better. the bass will be slightly better in terms of weight and agility. and the 458's can be upgraded to 468's.
the 108 Mk2 is in the same league mostly, and has a touch more musical texture and mid range tonal density, and is a bit more tube like....just a 'bit' more.. and it's not a long way behind the 458's in the other areas.
the greater grip of the 458 does deliver some slightly higher levels of nuance.
choice of speakers and room size also might be a factor. the 108 Mk2 can handle most any speaker, but there are degrees of 'handle'. the 458's can work with most any speaker and room size.
the 458's are my second most favorite amp, the 108 Mk2 my third most favorite. either would be a stellar choice.