I'm not a fan of the 6550. At worst, it's a cold, harsh sounding, clinical tube. At best, it sounds relatively warm, yet flat and uninvolving.
I should say that my experience with the KT77 consists of the JJ and Shuguang varieties, and not the classic, Holy Grail vintage British variety. Along those lines, the KT77 is somewhere between an EL34 and a KT88. I would obviously recommend it over the 6550. The KT77 has a pretty fat mid/upper bass that rivals the KT88, and can be great in some amps/systems, though too boomy in others. Its midrange is more open and clear than a KT88, though it will not compete with the better EL34 variants.
Personally, I find the EL34 best for my tastes, but if I had to award a second prize, it would be the KT77. I see a lot of audiophiles on these threads actually prefer the KT77. To me, it comes down to midrange, and thankfully, I've found a way with my amplifiers to produce the bass of the KT77/KT88 without losing that magical midrange the EL34s I use produce.