Stronglight 49D was one of the first (the first?) alloy crankarms sometime around 1934. I haven't heard of specific problems with these. You can use still available TA rings for these. The fact that they were available with little changes for 40 years either shows that people bought them despite their flaws, or that they were a pretty good design. I think the latter is the case. If you count in the TA Cyclotourist cranks, the same basic design continues to be available today.
Both Stronglight cranks should ride just fine.
I'd say upgrading a Peugeot isn't a crime, especially since going back to original equipment would be easy if you decided to do so in the future.
Anybody ever seen the steel Stronglight cranks that look almost like 49D alloys ones? Vintage Bicycle Quarterly 2 will show them on a René Herse in an old photo, but I've never seen one in real life!
Jan Heine, Seattle (working on the layout, hopefully going to print in 10 days)
http://www.mindspring.com/