Yeah – the M123's looks good & the other Shimano's I've got are pretty comfy.
I've always had size 41 shoes and not had a problem, so will probably stick to that I think.
With regard to how comfortable are these type of shoes for all day etc; I've found my Pearl Izumi's to be as comfortable as the more trail styled Shimano's I've got. You can't walk about in them quite as well, but not generally an issue. Even hike-a-bikes up Spanish hillsides weren't really an issue.
I would quite like to try the Diadora, but I also have no idea of how good they'd be, how their sizing comes up etc. and I am a bit wary of that ratchet strap.
Mate of mine has got Spesh BG shoes and rates them, but the only ones that'd fit in my budget are the £70 Sport ones which IMO looks a bit bland, although granted isn't the most important consideration.