KT88 circuitry vs. EL34

I've seen some cases where the same brand of amp using either KT88 or EL34. For example Quicksliver. What is the differentce between these tubes and certainly advantages/disadvantages b-n each other. Another words can I just plug into the same socket KT88 instead of EL34 or vice versa?
quick response,

If your amp manual/manufacturer says it accepts both then experiment.
just went thru testing several amps with el34 / kt88 configurations.
Both same brand jj tubes, the kt88 are bolder sounding with plenty power,transient better bass.
the EL34 gives nicer,more immediate real life midrange,and excels with acoustic/vocals.
So in a nutshell it will depends on the type of music you prefer to listen to the most.
As for me I settled for the el34 because of my type of music .

generally speaking:

EL34 for acoustic/vocals/quartets/(light rock)
KT88 for classical/(demanding rock/music)

I sort of agree with Thierry, but would like to add that the truly great KT88 tubes are all but impossible to find. Gold Lion, Genelex KT88's are wonderful, but the new replacements from Russia and China are boring when compared to the original.

On the other hand, the JJ Tesla (mentioned above) is a great tube. Used Amperex and Mullards are the best and can approach the original KT88's, even in it's strong areas.

With patience you can score good used EL34's for as little as $20.00. Decent Genelex (not completely worn out) go for $100.00. If you found brand new ones, the price would look like an error it would be so high.

Check that the manufacturer gives their blessing for use of both EL34 and KT88 before making your test. The tubes bias differently and draw different plate voltage. In some high voltage Triode configurations you can fry an EL34 quickly.

The EL 34 is actually a beam Pentode. One current amp that mates the EL34 correctly is the Wolcott Presence. Consistently getting 245 ( plus ) watts from only eight outputs. An EL 34 can produce terrific power if the amp is designed to match it properly.