Assuming that you own them both, it doesn't really matter. I'd go for La Chouffe first in general, although it depends on which beer (from each brewery) you've actually bought....
Is it a standard bottle of Duvel (33cl 9% abv) ? Is the Chouffe the 75cl (8% abv) bottle ? If it was me I'd go for the Chouffe as it's a bit less dry to my taste than Duvel - and I certainly would if it was the 75cl bottle ! Be careful, though, don't get sunburn ! Therein's the danger in drinking strong beers on sunny days !
Gone for the Duvel 33cl (at 8.5% on the label was I robbed??) ... Quite cool here today (though sunny) Just polished off a large Italian craft beer 75 cl prior to that from the local abbey so got to be careful as I am about to grate some potatoes for potato pancakes for dinner. (Watch my fingers!!) :-( Tough this retirement lark that 'nos!!
La Chouffe every day of the week. My favourite drink on the planet. Although I find Duvel palatable its not a patch on the taste of the Gnome Poison.
I'm actually retiring a week on Thursday (30 June). Looking forward to more leisure time. Clearly I need to moderate on the beer though - as you rightly point out Duvel is 8.5% - my rate of brain cell destruction is affecting my memory ! I do enjoy the Belgian abbey beers though. Enjoy your beer and pancakes !