I've never done work with stone, but on steel the higher wattages only come in handy when you're really beating on the material. I own a 1200 watt Hitachi, and a 600 watt cheapo unit from Harbor Freight, and 90% of the time I prefer the cheap unit because it's like 3x lighter than the Hitachi, and I don't think I've ever stalled either unit, although the little one does get quite hot when I push it hard.
The weight difference between the two linked Bosch units isn't much (100g), so IMO it comes down to design and aesthetics like the positioning of the switch. All of the Metabo and Bosch units have the tiny click over switches, whereas some of the "American" brands have triggers much further down the handle (like this
one), which might be more pleasant for long sessions. I can't find any of them on the German Amazon though.
Definitely think about dust collection and removal because stone dust can be super terrible for your lungs. Maybe even something like this
if she's doing mostly flat grinding work.