if you're retired and will be traveling for fun then who cares if you break down. fix up the 91 and enjoy.

when driving my 1987 300SDL i am happier then when driving my fathers 2001 S500.

This in my opinion is a similar trade off. Sure a newer SL would be faster smoother, and more reliable, but its all a personal choice. And i like the older one.

Drive the newer SL and then think back as to what your 91 was like and try to imagine what would make you more happy.

If you have any sentimental connection your current SL you'll want her with you on the travels. I see it as going on a trip with an old friend or a business associate.