A fragile front tooth from a root canal from 40 years ago. Should I now get a crown or veneer on that tooth?

Lilwl,  my suggestion to you  is to decide  what you want for your smile . A veneer will not have enough material to cover a dark tooth color.  A crown of the right material will be able to cover that dark tooth.  A veneer on the neighboring tooth will improve the color match of your two front teeth.  And regarding losing to structure to a crown, crowns are regularly recommended for and placed on endodontically treated teeth in the front and in the back of the mouth.  Conserving tooth structure is only one of many considerations when choosing the appropriate restoration for a tooth. Others are, but are not limited to, strength and longevity and finally, aesthetics. 

I agreed with other two dentists answer's. It is true that root canal weaken your teeth and if you again getting treatment on the same tooth, it will effect other teeth more. As, you haven't share your x-ray and images with us which may be help us to provide you more suitable answer. 
But, yes crowing and after that veneer treatment is good for your teeth. It repairs the fracture and veneers is a cosmetic procedure that eliminates unsightly gaps, cracks, chips, discoloration, or slight imperfections.

Great questions and also difficult to talk over specific treatment without examining you and looking at x rays and talking over your esthetic goals..It sounds like you want to brighten some or all of your teeth--the first dentist with suggesting a crown and veneer is looking to protect and brighten the two front teeth-the second is looking at more of brightening all of your teeth and then matching the front two.If the tooth had a root canal 40 years ago obviously it has resisted fracture this long. Crowning a front tooth can strengthen and it can also weaken a front tooth.Like the other fine dentist from TX suggests, posts can also be a consideration when reinforcing front teeth with root canals.I am sorry if I have added more confusion to the mix. My guess is that esthetics is a concern and may be the determining factor.Good luck

Teeth are weakened when a root canal is done, but removing more tooth structure weakens the tooth even more. You don't mention if your teeth have posts placed, as this could give some added stability. Either treatment option will work. Don't have high expectations for improving the color of your dark teeth with external bleaching. Your other teeth will whiten but your root canal tooth won't whiten much. Without additional diagnostic information it is impossible to give you an answer that is specific to your situation.