Do NOS Mullard EL34 tubes last much longer than NOS SED (Svetlana) EL34 ?

That's an insane price for the Mullards. Who is quoting you that? 

Source: I buy and sell rare audio tubes full-time.
 

I have four SED "winged C" EL34's that came with my purchase of a Dynaco ST70. I doubt that original made-in-Britain Mullards will last any longer. Don't forget that the so-called NOS Mullards are actually present-day production from Russia.

forget life, either should last a long long time.

which sounds best to you? do you want to listen to the less preferred sound for a long long long time?

You could buy from a source that lets you return them, try the least expensive compared to your existing.

sound great, keep em. if not as good as your existing, return, get .......

forget life, either should last a long long time.

which sounds best to you? do you want to listen to the less preferred sound for a long long long time?

You could buy from a source that lets you return them, try the least expensive compared to your existing.

sound great, keep em. if not as good as your existing, return, get .......

This is the right advice. And don't buy tubes from anyone that won't let you return them for any reason within the first thirty days.