amplifier longevity

Modern caps are a lot better than what was out in the 1980s.  If you are buying new right now I'd expect either to last you a liftetime (minus tube life).

If you have vintage gear and can afford to get them recapped today, go ahead and you'll be done. :)

I think they are about the same.  Alot depends on how hot the amplifier runs I think.  I have heard that really hot amps cook the capacitors and make them age faster.

Too many factors involved. The design, the operating mode (Class A vs AB), parts quality. That also doesn't assume catastrophic tube failure, or shorting the speaker terminals of your SS amps.

 Under normal use, no reason either can't be completely reliable.

The exception to all of this is Class D. I am seeing an up tick in Class D failures of amps that are only 3-5 years old. Some have even become non-repairable due to obsolete parts or boards.

Depends on build quality does not matter tubes or ss..

Because I am a compulsive spender I tend not to keep equipment as long as I should. 

That being said the oldest amp I owned was a set of Manley Neo Classic 250s. I bought them used when they were 7 years old. 

I traded them in for a set of Class D monoblocks 3 years later because I couldn't stand the heat and the tube maintenance. 

At 10 years old they ran like champions and had not one issue.