Russian AC are not nearly as nicely made as western AC, particularly towards the back where it matters less as it's in dirtier air anyway. Russia was (WAS) way ahead of us with Titanium but there advantage there is long gone.
I doubt very much that the F-35 could hang with a modern Sukhoi when it get to a merge. In fact, I don't think the designers would argue that the F-35 is well suited to that task. If it isn't over after the first weapons release I'd say the advantage we have evens out and if it becomes a gun fight the F-35 is in trouble.
And saying the modern Sukhoi AC like the Su-35/37 are derivative of the Su-27, which started life nearly 40 years ago is, well, pretty unfair. In addition to there improvements in manufacturing and materials as well as advances they've made in design, they have dramatically improved there avionics though clearly well behind our newer efforts. They have also been doing thrust vectoring for a long time. Lastly, Russian AC have generally lost speed more slowly in turns giving them an energy advantage.
The fact that the SU-35 etc share lines and resemble the Su-27, a 40 year old AC, does not make the Su-35/37 40 year old AC.
Brian
Well, let's be careful here, I was only talking about F-35 airframe vs. Su-35. (Really, I guess, airframe + engine because it's very hard to consider the airframe apart from the engine). Essentially, which fighter would be able to get on the other's tail given a neutral starting position, and no weapons actually being fired. While I think that the F-35 could be equal or superior, I respect that the Su-35 could be better.
If you change that to an actual WVR engagement, I think the F-35 is the prohibitive favorite. It's 360 degree IR sensors, HOBS/LOAL missiles and helmet mounted sight give it a huge advantage. And yes, I know Russia has helmet sights too, but as far as I'm aware, nothing that compares to the AIM-9x. All that doesn't even take into account that with its low observables and superior sensors, the F-35 will likely enter the merge with a tactical advantage.
The Su-35, just like the F/A-18 E&F, the F-15K, and the later F-16 blocks, is a derivative of a '70s design. I'm not sure how that's unfair.