To start with - what's wrong with doing both!
Which is best really depends on what you personally are looking for/interested in.
Le Mans 24hr
Very big, very packed, very drunken, very loud (the cars), very good laugh. Oh, and you get to see some very expensive metal on the road down/back -
much of it stopped paying speeding fines to the local policeman.
It's best to treat it as a week sat in a field full of (happy) drinking Brits and Germans, with a race thrown in.
Camping tends to fill up fast and, with the exception of Houx is first come first served. Get down early (Wed./Thurs.) and tape out your plot and your
fine. Turn up on Fri./Sat. and you have to squeeze in where you can.
The atmosphere, both around the circuit and in the camp sites is great. It's so busy, it's infectious. Add to that watching LMP1 cars glowing their
break disks at 5am for Arnage corner, while you have a beer in one hand and a coffee in the other and you will have a great time.
Le Mans Classic
Quite small (compaired to LM24), but no less friendly.
You don't get 24hrs continuous racing, but something like 8 age based classes with a break between each class (which I found a little annoying
personally).
What you do get are loads of classic cars, both on and off track.
Either way you get a good few days sat in a (hopefully) sunny French field drinking beer and relaxing