I read this description of 6550's versus KT88's once on some tube site and I find it is pretty accurate.
They described the KT88 as a larger big bottle EL34 that can handle higher plate voltages but still has some warmth and emphasis on mids like an EL34, supposedly it is why Marshall chose the KT88's for the Marshall Majors and why Jimmy Page had KT88's installed in his Marshall's at one point.
The same site described 6550's as a big big bottle 6L6 that can handle more plate voltage and more rugged, 6550's are quite linear and balanced in the sonic spectrum much like a 6L6 and my ears also agree with that description.
So if you like EL34's the KT88's might float your boat and if you like 6L6's then the 6550's might be your tube.
The only new production KT88 I did not like was the JJKT88 it was just too warm and undefined soft in the attack, but some people really love them. I like the Sovtek, SED, and EH KT88's all of those sound great and usually are reliable. As for 6550's try to find some NOS SED's or even GE6550A's but the new New Sensor EH's and Svetlana are pretty decent. I've been running older SED and Svetlana 6550B and C's for years and they are great sounding tubes and very reliable.
Here's the description of the JJKT88 from Doug's tubes and it is accurate and why I didn't care for them.
https://dougstubes.com/collections/6550-kt88-kt90/products/ruby-kt88-cz
"The Ruby KT88-CZ has a tightly compressed bottom end that is perfect for jazz and audio. Just like the JJ KT88, repackaged by Ruby Tubes."
If you like a compressed bottom end then you might like the JJKT88.
Stay away from the Valve Arts 6550 A tubes, they were supposed to be a copy of the GE6550A's and the couple of sets I tried went bad quickly and one or two started to red plate and or shorted completely out. I won't run another set of them. There are people selling old NOS new or tested used SED's/Winged C's on Ebay every now and then and they are my favorite.
KT88's and 6550 should bias in the same range.